No. 16-402
================================================================

In The

Supreme Court of the United States
-----------------------------------------------------------------TIMOTHY IVORY CARPENTER,
Petitioner,
v.
UNITED STATES OF AMERICA,
Respondent.
-----------------------------------------------------------------On Writ Of Certiorari To The
United States Court Of Appeals
For The Sixth Circuit
-----------------------------------------------------------------BRIEF OF PROFESSOR ORIN S. KERR AS
AMICUS CURIAE IN SUPPORT OF RESPONDENT
-----------------------------------------------------------------ORIN S. KERR
Counsel of Record
2000 H Street, NW
Washington, DC 20052
(202) 994-4775
okerr@law.gwu.edu
================================================================
COCKLE LEGAL BRIEFS (800) 225-6964
WWW.COCKLELEGALBRIEFS.COM

Electronic copy available at: https://ssrn.com/abstract=3047300

 i
TABLE OF CONTENTS
Page
TABLE OF CONTENTS ........................................

i

TABLE OF AUTHORITIES ...................................

iii

INTEREST OF THE AMICUS CURIAE ...............

1

SUMMARY OF ARGUMENT ................................

1

ARGUMENT ...........................................................

2

I. COLLECTION OF HISTORICAL CELLSITE DATA IS UNPROTECTED BY THE
FOURTH AMENDMENT BECAUSE IT IS
THE NETWORK EQUIVALENT OF OBSERVATION IN PUBLIC SPACE...............

3

II.

THE BALANCE OF PRIVACY AND SECURITY IN HISTORICAL CELL-SITE RECORDS
IS BEST RESOLVED BY LEGISLATION
SUCH AS THE STORED COMMUNICATIONS ACT ..................................................

7

(a) Cell Phones Can Be Used to Facilitate
Crime .....................................................

9

(b) Encryption Limits Government Communications Surveillance Power .......... 11
(c) Congress and State Legislatures are Actively Engaged in Deciding the Proper
Statutory Protection For Cell-Site Records......................................................... 12
III.

THE COURT SHOULD REJECT CARPENTER’S MOSAIC THEORY OF THE FOURTH
AMENDMENT ............................................. 15

Electronic copy available at: https://ssrn.com/abstract=3047300

 ii
TABLE OF CONTENTS – Continued
Page
IV. WHETHER REASONABLE PEOPLE EXPECT PRIVACY IN CELL-SITE RECORDS IS IRRELEVANT, AS THE THIRD
PARTY DOCTRINE IS ABOUT MANIFESTING SUBJECTIVE EXPECTATIONS
OF PRIVACY................................................ 20
V.

THE ENACTMENT OF PRIVACY LEGISLATION DOES NOT GOVERN OR INFLUENCE THE MEANING OF THE FOURTH
AMENDMENT ............................................. 26
(a) Privacy Legislation Does Not Provide
Evidence of Reasonable Expectations of
Privacy ................................................... 26
(b) 47 U.S.C. § 222 Does Not Make Cell-Site
Records the User’s Fourth Amendment
“Papers”.................................................. 29

CONCLUSION .......................................................

30

Electronic copy available at: https://ssrn.com/abstract=3047300

 iii
TABLE OF AUTHORITIES
Page
CASES
Berger v. New York, 388 U.S. 41 (1967) ......................11
Dunaway v. New York, 442 U.S. 200 (1979) ...............15
Ex Parte Jackson, 96 U.S. 727 (1878) ...........................7
Hester v. United States, 265 U.S. 57 (1924) ................21
Hoffa v. United States, 385 U.S. 293 (1966) ...... 21, 22, 23
Johnson v. United States, 333 U.S. 10 (1948).............10
Katz v. United States, 389 U.S. 247 (1967) ......... passim
Lewis v. United States, 385 U.S. 206 (1966) ...............21
Lopez v. United States, 373 U.S. 427 (1963) ...............22
Minnesota v. Carter, 525 U.S. 83 (1998) .....................29
New York v. Belton, 453 U.S. 454 (1981) ............... 15, 19
On Lee v. United States, 343 U.S. 747 (1952) .............22
Osborn v United States, 385 U.S. 323 (1966) .............22
Riley v. California, 134 S. Ct. 2473 (2014).... 8, 13, 15, 20
Rios v. United States, 364 U.S. 253 (1960) .................21
Smith v. Maryland, 442 U.S. 735 (1979) ......................7
State v. Gray, 234 N.C. App. 197 (2014) .....................10
Terry v. Ohio, 392 U.S. 1 (1968) ....................................9
United States v. Ackerman, 831 F.3d 1292 (10th
Cir. 2016) .................................................................29
United States v. Dionisio, 410 U.S. 1 (1973) .................3

 iv
TABLE OF AUTHORITIES – Continued
Page
United States v. First National Bank, 295 F. 142
(S.D. Ala. 1924), aff ’d, 267 U.S. 576 (1925)...............7
United States v. Hill, 2013 WL 11317131 (N.D.
Okla. 2013) ..............................................................10
United States v. Horton, 863 F.3d 1041 (8th Cir.
2017) ..........................................................................9
United States v. Jacobsen, 466 U.S. 109 (1984)..........25
United States v. Jeffers, 342 U.S. 48 (1951)................21
United States v. Jones, 565 U.S. 400 (2012) ......... 15, 16
United States v. Olusola, 564 Fed.Appx. 466
(11th Cir. 2014)........................................................10
United States v. Supreme Court of N.M., 839
F.3d 888 (10th Cir. 2016) .........................................28
CONSTITUTIONAL PROVISIONS
Supremacy Clause ......................................................28
U.S. Const. Amend. IV ........................................ passim
STATUTES
18 U.S.C. § 2702 .................................................... 12, 24
18 U.S.C. § 2703 ..........................................................24
18 U.S.C. § 2703(d) ................................................ 12, 20
47 U.S.C. § 222 ...................................................... 26, 29

 v
TABLE OF AUTHORITIES – Continued
Page
Cal. Penal Code
§ 1546(d) ..................................................................14
§ 1546.1(b)(1) ...........................................................14
§ 1546.1(d) ...............................................................14
Pub. L. No. 103-414, Title II, § 207(a) (1994) .............13
OTHER AUTHORITIES
Berin Szóka, Privacy Reform Finally Moves in
House: Goodlatte Promises Geolocation Privacy Bill Will Move Soon, Tech Policy Corner,
Apr. 14, 2016 ............................................................14
EFF Statement on and Analysis of Digital Telephony Act, October 8, 1994 ...................................13
Eugene Volokh, Freedom of Speech and Information Privacy: The Troubling Implications of
a Right to Stop People from Speaking About
You, 52 Stan. L. Rev. 1049 (2000) ...........................30
Jake Swearingen, No, the CIA Hasn’t Compromised Signal and WhatsApp, N.Y. Mag. Select
All, Mar. 7, 2017 ......................................................11
Orin S. Kerr, A User’s Guide to the Stored Communications Act, And A Legislator’s Guide to
Amending It, 72 Geo. Wash. L. Rev. 1208
(2004) .......................................................................12
Orin S. Kerr, The Mosaic Theory of the Fourth
Amendment, 111 Mich. L. Rev. 311 (2012) ....... 15, 17

 vi
TABLE OF AUTHORITIES – Continued
Page
Orin S. Kerr & Bruce Schneier, Encryption Workarounds, Geo. L.J. (forthcoming 2018), available at https://papers.ssrn.com/sol3/papers.cfm?
abstract_id=2938033 ...............................................12
Orin S. Kerr, An Equilibrium-Adjustment Theory of the Fourth Amendment, 125 Harv. L.
Rev. 476 (2011) ................................................ 7, 8, 11
Orin S. Kerr, Applying the Fourth Amendment to
the Internet: A General Approach, 62 Stan. L.
Rev. 1005 (2010) ................................................ 4, 5, 6
Orin S. Kerr, Four Models of Fourth Amendment
Protection, 60 Stan. L. Rev. 503 (2007) ...................20
Orin S. Kerr, Katz Has Only One Step: The Irrelevance of Subjective Expectations, 82 U. Chi.
L. Rev. 113 (2015) ........................................ 21, 23, 24
Orin S. Kerr, The Effect of Legislation on Fourth
Amendment Protection, 115 Mich. L. Rev. 1117
(2017) ........................................................... 27, 28, 29
Patricia M. Wald, Some Observations on the Use
of Legislative History in the 1981 Supreme
Court Term, 68 Iowa L. Rev. 195 (1983) .................28
Susan Freiwald, At the Privacy Vanguard: California’s Electronic Communications Privacy
Act (CalECPA), Berkeley Tech. L.J. (forthcoming 2018) ..................................................................14

 1
INTEREST OF THE AMICUS CURIAE
Orin S. Kerr is the Fred C. Stevenson Research
Professor at the George Washington University Law
School. Beginning in January 2018, he will be a Professor of Law at the University of Southern California
Gould School of Law. The interest of amicus is the
sound development of Fourth Amendment law.1
------------------------------------------------------------------

SUMMARY OF ARGUMENT
The Fourth Amendment does not apply to the collection of historical cell-site records. Collection of historical cell-site records is the network equivalent of
unprotected observation in public space. Because the
effect of technological change is uncertain and evolving, the Court should allow Congress and state legislatures to continue their active and dynamic debates
about the proper regulation of historical cell-site records. The Court should also reject Carpenter’s mosaic
theory of the Fourth Amendment. The third-party doctrine should be retained, but the Court should restore
it to its proper place as the subjective expectation of
privacy test.
------------------------------------------------------------------

1

All parties have consented in writing to the filing of this
brief. No entity or person aside from amicus curiae made any
monetary contribution supporting the preparation or submission
of this brief. No counsel for any party to this proceeding authored
this brief in whole or in part.

 2
ARGUMENT
This is a challenging case. The facts involve new
and developing technology. The law features vague and
often-criticized tests. And the Court can’t help but feel
pulled by two competing and legitimate concerns. On
one hand, the law must keep up as technology changes
to maintain privacy protections. On the other hand, the
blunt instrument of the Fourth Amendment shouldn’t
be forced beyond its proper role.
This brief offers a way through the difficult issues
in five steps. It begins by anchoring the legal issues
raised in the familiar context of the physical world. It
then considers whether technological change justifies
a departure from the Court’s traditional rules. After
that, it explains why the Court should reject the theory
Carpenter advocates that would draw a distinction between longer-term and shorter-term surveillance. It
next explains how the parties have misunderstood a
significant part of the case, and how the case becomes
much simpler when properly understood. The brief
concludes by discussing the proper relationship between the interpretation of the Fourth Amendment
and statutory law.

 3
I.

COLLECTION OF HISTORICAL CELL-SITE
DATA IS UNPROTECTED BY THE FOURTH
AMENDMENT BECAUSE IT IS THE NETWORK EQUIVALENT OF OBSERVATION
IN PUBLIC SPACE.

Obtaining historical cell-site records from a cell
phone provider is like obtaining testimony from an
eyewitness to suspicious conduct. By contracting with
a cell phone network provider to deliver their calls, customers ensure that network providers may be available to testify – whether in person or by sending records
– about how the providers made that delivery for their
users. Just as a person voluntarily exposes himself to
observation by traveling in public to deliver a communication, so does a person voluntarily expose himself to
observation by hiring an agent to deliver his communications remotely. The Fourth Amendment is not implicated by compelling testimony from an eyewitness
or by observation in public. See United States v. Dionisio, 410 U.S. 1, 8-10 (1973); Katz v. United States, 389
U.S. 247, 361 (1967) (Harlan, J., concurring). The same
rule should apply in the analogous context of obtaining
historical cell-site records.
To appreciate this perspective, it is essential to
realize that the Fourth Amendment traditionally
achieves a balance between protected and unprotected
conduct. On one hand, the Fourth Amendment extends
constitutional protection to a person’s “houses, papers,
and effects” from unwarranted government interference. U.S. Const. Amend. IV. On the other hand, the

 4
Fourth Amendment offers no protection from government surveillance in public. See Katz, 389 U.S. at 361.
A useful approximation of the Court’s many cases
is that the Fourth Amendment protects what occurs
inside but doesn’t protect what occurs outside. See
Orin S. Kerr, Applying the Fourth Amendment to the
Internet: A General Approach, 62 Stan. L. Rev. 1005,
1009-11 (2010) (hereinafter General Approach). This
dividing line strikes an essential balance in physicalworld investigations. It limits government power to
protect privacy in some spaces (inside), but it allows
the government to investigate without restriction in
other spaces (outside). See id. at 1011.
When applying the Fourth Amendment to communications networks, the Court’s first instinct should be
to preserve this essential traditional balance. To ensure that the balance of the Fourth Amendment is
maintained in our increasingly networked technological world, the Court should translate the law’s treatment of physical world space across the shift to a
networked environment. This is necessary to maintain
the traditional degree of privacy protection the Fourth
Amendment offers as technology changes.
Translation produces a simple rule: Although the
Fourth Amendment protects the contents of communications sent over a network, it does not protect noncontent addressing information used to deliver those
contents. See General Approach at 1017-22. Judge
Kethledge properly recognized in his opinion below
that the Court’s precedents have adopted the content/

 5
non-content distinction in its caselaw on postal letters
and telephone calls. Pet. App. 9a-13a. But the content/
non-content line is essential for a deeper reason beyond precedent. It recreates the inside/outside distinction from the physical world, protecting the network
equivalent of inside surveillance (contents) and leaving unprotected the network equivalent of outside surveillance (non-content records). See General Approach
at 1017-22.
To see this, imagine a world without communications networks. If Alice wants to communicate with
Bob, Alice has to leave her home and travel to Bob’s
house. If the police suspect that Alice and Bob are
conspirators planning a crime, and they assign an officer to watch Alice’s whereabouts, the police can collect only some information without triggering the
Fourth Amendment. The police cannot learn the contents of what Alice and Bob said inside Bob’s home
without a warrant. On the other hand, the police can
observe Alice and see what she did in public – when
she left home, where she traveled, when she arrived at
Bob’s house, and where they both live – without triggering the Fourth Amendment.
Next imagine that Alice calls Bob on her cell phone
instead of meeting him in person. Alice no longer has
to travel to meet Bob. The cell phone network delivers
the call from Alice to Bob, making a remote transfer
that eliminates the need for a public trip. But, critically, the same information exists. What was previously the contents of the conversation in Bob’s house is
now the contents of the phone call between Alice and

 6
Bob. And what was previously Alice’s publicly observable trip from her house to Bob’s house is now a record
that the phone provider generated and may keep about
when the call was made, to and from what numbers,
and what cell towers were used to deliver it.
To maintain the balance of the Fourth Amendment, courts should treat the same information in the
same way in both the physical and network contexts.
The contents of phone calls should be protected, as
they are the telephone equivalent of protected inside
space. This means, in the Internet context, that the
contents of e-mails, text messages, and files that users
place in cloud storage should receive full Fourth
Amendment protection. See General Approach at 1029.
On the other hand, non-content records generated by
network providers – the business records they generate about how they delivered the communications –
should not be protected because they are the network
equivalent of the publicly observable trip that is outside such protection in the physical world. See id. at
1017-22.
It is true, as Carpenter argues, that cell phones are
“indispensable for full participation in family, social,
professional, civic, and political life.” Petr. Br. at 40. But
that provides no more reason to protect cell-site records than does the normal human need to venture outside provide reason to protect observation in public.
The Fourth Amendment extends protection to some aspects of life but leaves other parts unprotected both in
the physical world and in the network environment.

 7
This approach should lead the Court to reaffirm
the traditional rule, over a century old, that collection
of non-content addressing information for a network
communication does not implicate the customer’s
Fourth Amendment rights. See Ex Parte Jackson, 96
U.S. 727, 733 (1878) (postal letters); Smith v. Maryland, 442 U.S. 735, 743-44 (1979) (telephone calls). See
also United States v. First National Bank, 295 F. 142,
143 (S.D. Ala. 1924) (rejecting a challenge to government access to a couple’s entire bank records, stating
that “[t]his is not a question of a search and seizure of
a party’s books and papers, but of whether a witness
who has information as to a party’s dealings may be
required to testify to those facts”), aff ’d, 267 U.S. 576
(1925) (per curiam).
II.

THE BALANCE OF PRIVACY AND SECURITY
IN HISTORICAL CELL-SITE RECORDS IS
BEST RESOLVED BY LEGISLATION SUCH
AS THE STORED COMMUNICATIONS ACT.

But wait, Carpenter says: Cell phone technology
has expanded government power to invade privacy.
In petitioner’s view, changing technology justifies adding new protection for cell-site records to restore the
prior level of government power. Petr. Br. at 14-21. I
have called this argument “equilibrium-adjustment.” I
agree with Carpenter that, in a proper case, equilibriumadjustment is an appropriate way to update Fourth
Amendment rules in light of technological change. See
generally Orin S. Kerr, An Equilibrium-Adjustment
Theory of the Fourth Amendment, 125 Harv. L. Rev. 476,

 8
525-42 (2011) (hereinafter Equilibrium-Adjustment).
Technological change that dramatically expands government power under old legal rules can justify imposing greater privacy protection. See, e.g., Riley v.
California, 134 S. Ct. 2473, 2494 (2014) (requiring a
warrant to search a cell phone incident to arrest because of “all they contain and all they may reveal”);
Equilibrium-Adjustment at 525-42.
This is not such a case, however. First, Carpenter
exaggerates the threat to privacy posed by historical
cell-site records. Such records ordinarily cover too
broad an area to be particularly revealing on their own.
Resp. Br. at 24-28. Notably, Carpenter does not say
what personal or sensitive fact was learned about him,
other than his location generally near a string of robberies, when the government collected his records.2
Carpenter’s brief instead focuses on the privacy threat
raised by different location technologies such as GPS

2

The Technology Experts claim that the records show Carpenter “attended a particular church in Detroit nearly every Sunday.” Brief of Amici Curiae Technology Experts at 29 n.49. That is
wrong. According to an amicus brief filed in the Sixth Circuit, Carpenter told amicus counsel that he attended a particular church.
See Brief of Amicus Curiae Am. Civil Liberties Union et al. at 11,
819 F.3d 880 (2016) (Nos. 14-1572 & 14-1805), available at https://
www.aclu.org/legal-document/united-states-v-carpenter-amicus-brief.
Analysis of the cell-site records then showed that on “a number of
Sundays,” Carpenter’s phone was used in sectors that included
that church. Id. This provides no basis to conclude that the records, considered alone, would have revealed church attendance.
The reason to think Carpenter attended a particular church is
that he said so.

 9
tracking. See Petr. Br. at 19-29, 42-47. Those technologies typically require access to information stored inside a person’s physical device, however, and therefore
their use may be searches under traditional Fourth
Amendment principles. See, e.g., United States v. Horton, 863 F.3d 1041, 1047 (8th Cir. 2017).
More fundamentally, technology’s impact on government communications surveillance power is decidedly mixed. Although cell phone technology expands
government power in some ways, it shrinks government power in other ways that Carpenter ignores. A
close look at two dynamics – how cell phones can facilitate crime and the role of encryption – suggests that
the overall impact of technology on government surveillance power is uncertain and still evolving.
(a) Cell Phones Can Be Used to Facilitate
Crime.
Cell phone technology can limit government power
by giving wrongdoers a powerful new way to avoid detection in the commission of crime. Consider the impact of cell phones on a group robbery such as the one
in this case. Before the cell phone age, conspirators
planning to rob a store had to “case the joint” in the
open and meet in person to coordinate their plans.
Their suspicious behavior risked drawing the attention of officers nearby. See Terry v. Ohio, 392 U.S. 1
(1968) (permitting an officer to “stop and frisk” conspirators acting suspiciously in front of a store in the
course of planning a robbery).

 10
Cell phones make such crimes easier to commit
and harder to detect. These days, robbery conspirators
can coordinate their crimes silently and instantly over
any distance by cell phone text message. One conspirator can watch the store and text his observations to the
others. See, e.g., State v. Gray, 234 N.C. App. 197, 206
(2014). A second conspirator can serve as a lookout and
text the group if danger appears. See United States v.
Olusola, 564 Fed.Appx. 466, 468 (11th Cir. 2014). After
the robbery, the leader can text the rest with instructions about where to meet to divvy up the loot. See, e.g.,
United States v. Hill, 2013 WL 11317131, *2 (N.D.
Okla. 2013). The cell phone replaces awkward and suspicious in-person meetings with concealed, silent, and
instant coordination. An officer walking the beat will
be none the wiser. The conspirators will look like everyone else: Just people checking their phones.
The ways that cell phones can facilitate crime and
avoid detection counsels against creating new Fourth
Amendment protections for cell phone records. Obviously, most people don’t use their phones to commit
crimes. But most people don’t have their records collected by court order under the Stored Communications Act, either. The key point is that the effect of cell
phone technology on the “often competitive enterprise
of ferreting out crime,” Johnson v. United States, 333
U.S. 10, 14 (1948), operates as a two-way street. The
ability of cell phone companies to deliver communications quickly and silently over any distance cuts both
ways. It can lead to records about the delivery that
helps the police, and it can aid in the commission of

 11
crime that helps wrongdoers. Both should be considered. See Equilibrium-Adjustment at 512-17.
(b) Encryption Limits Government Communications Surveillance Power.
The rise of encryption also complicates the dynamic
Carpenter describes. In the past, the government ordinarily had the technical capacity to intercept the contents of phone calls and communications. See Berger v.
New York, 388 U.S. 41, 46 (1967). In the last decade,
however, software providers have provided powerful
and free technologies that enable anyone to shield
the contents of their communications from lawful government access by encrypting the contents of their
network communications during transmission. For example, today over one billion people worldwide use
“WhatsApp,” a program first introduced in 2009 that
encrypts messages end-to-end. See Join WhatsApp.3 As
far as the public knows, not even the Central Intelligence Agency can break WhatsApp’s encryption. See
Jake Swearingen, No, the CIA Hasn’t Compromised Signal and WhatsApp, N.Y. Mag. Select All, Mar. 7, 2017.4
Doing so is presumably beyond the capacity of federal,
state, or local law enforcement.
Encryption shows how the impact of technology on
government communications surveillance power can
work both ways. Encryption brings many wonderful
3

https://www.whatsapp.com/join/
http://nymag.com/selectall/2017/03/no-the-cia-hasnt-crackedencrypted-chat-app-signal.html
4

 12
benefits, but it can leave investigators unable to read
a target’s encrypted communications even with a
search warrant. It is too early to tell how far encryption
will interfere with government investigative powers.5
But because users generally can’t encrypt non-content
records such as historical cell-site records, the collection of such records may take on a more important role
in future surveillance practices. The Court should be
reluctant to introduce new constitutional protections
for non-content records when the existing constitutional framework for access to contents may be impeded by new encryption technology.
(c) Congress and State Legislatures are Actively Engaged in Deciding the Proper
Statutory Protection For Cell-Site Records.
Because the effect of technology on communications surveillance is a mixed bag, the Court should
continue to allow legislatures to debate and decide
how much protection cell-site records should receive.
The federal Stored Communications Act sets a floor for
all investigators. See 18 U.S.C. § 2702, § 2703(d). See
generally Orin S. Kerr, A User’s Guide to the Stored
Communications Act, And A Legislator’s Guide to
5

Law enforcement can try several ways to work around encryption to access contents. See generally Orin S. Kerr & Bruce
Schneier, Encryption Workarounds, Geo. L.J. (forthcoming 2018),
available at https://papers.ssrn.com/sol3/papers.cfm?abstract_id=
2938033. These methods may or may not work, however, and some
require considerable resources beyond the reach of many criminal
investigations. See id. at 33-35.

 13
Amending It, 72 Geo. Wash. L. Rev. 1208, 1209-25
(2004) (summarizing the privacy protections of the
Stored Communications Act). The states are free to impose greater restrictions on state and local officers. The
Court should not short-circuit this active and dynamic
legislative process by imposing a one-size-fits-all constitutional rule. See Riley, 134 S. Ct. at 2497-98 (Alito,
J., concurring in the judgment).
Legislators certainly recognize the need for some
kind of statutory privacy protection limiting government access to cell-site records. Everybody gets that.
The real debate is over what standards and remedies
should govern. In 1994, Congress amended the Stored
Communications Act to impose a new “specific and articulable facts” standard for access to non-content historical network account records that include cell-site
records. See Pub. L. No. 103-414, Title II, § 207(a)
(1994). At the time, amicus curiae Electronic Frontier
Foundation celebrated the new privacy law as a “critical” measure that achieved “a significantly greater
level of protection” than traditional network privacy
laws. See EFF Statement on and Analysis of Digital Telephony Act, October 8, 1994.6
Since 1994, Congress has frequently debated
whether to further raise the statutory standard for access to cell-site records. By my count, the House Judiciary Committee and its subcommittees have held

6
https://w2.eff.org/Privacy/Surveillance/CALEA/digtel94_passage_
statement.eff

 14
hearings partly or largely about this question five different times – in 2000, 2010 (twice), 2012, and 2013.7
Committee Chair Robert Goodlatte promised last year
that more legislative attention was coming. See Berin
Szóka, Privacy Reform Finally Moves in House: Goodlatte Promises Geolocation Privacy Bill Will Move
Soon,8 Tech Policy Corner, Apr. 14, 2016.
State legislatures have also been active. California
recently enacted the most far-reaching statutory privacy protection for network and electronic communications ever seen in the United States. See generally
Susan Freiwald, At the Privacy Vanguard: California’s
Electronic Communications Privacy Act (CalECPA),
Berkeley Tech. L.J. (forthcoming 2018).9 Starting in
2016, the law not only requires a warrant for California law enforcement access to non-content records,
including historical cell-site data, but it also imposes
special particularity and nondisclosure rules for those
warrants. See Cal. Penal Code § 1546(d), § 1546.1(b)(1),
§ 1546.1(d).

7

The published reports of these hearings are available

here:
bit.ly/HouseReport2000
bit.ly/HouseReport20101
bit.ly/HouseReport20102
bit.ly/HouseReport2012
bit.ly/HouseReport2013
8
https://techpolicycorner.org/email-privacy-reform-finally-movesin-house-df8f09962de6
9
https://papers.ssrn.com/sol3/papers.cfm?abstract_id=2939412

 15
III. THE COURT SHOULD REJECT CARPENTER’S MOSAIC THEORY OF THE FOURTH
AMENDMENT.
Carpenter chiefly argues that the Court should
follow the concurring opinions in United States v.
Jones, 565 U.S. 400 (2012), which suggested that certain kinds of long term surveillance can become a
Fourth Amendment search. Petr. Br. at 14-32. The
Jones concurring opinions reflect the so-called “mosaic
theory,” by which government evidence-collection is
not a search in isolation but becomes a search when
aggregated and analyzed over some period of time to
create a mosaic picture of a person’s activities. See generally Orin S. Kerr, The Mosaic Theory of the Fourth
Amendment, 111 Mich. L. Rev. 311 (2012) (hereinafter
Mosaic Theory).
A majority opinion of the Court has never adopted
the mosaic theory. And it should not start now. The
mosaic approach is well-intentioned but deeply misguided. It is a dramatic departure from traditional approaches, and it would drag state and federal courts
into impossible line-drawing exercises that would
cause endless confusion. Adopting the mosaic theory
“would keep defendants and judges guessing for years
to come,” Riley, 134 S. Ct. at 2493, instead of providing
the bright-line rules that are “essential to guide police
officers.” New York v. Belton, 453 U.S. 454, 458 (1981)
(quoting Dunaway v. New York, 442 U.S. 200, 213-14
(1979)).

 16
Before discussing the problems with the mosaic
theory, it’s important to realize that the Jones concurrences themselves suggest it may not apply to collection of historical cell-site records. Both concurrences in
Jones were premised at least in part on the absence of
legislation limiting the executive branch. See Jones,
565 U.S. at 416 (Sotomayor, J., concurring) (suggesting
that the appropriateness of permitting executive discretion should be considered “in the absence of any
oversight from a coordinate branch”); id. at 430 (Alito,
J., concurring in the judgment) (justifying the approach because legislatures “have not enacted statutes
regulating the use of GPS tracking technology for law
enforcement purposes”). As discussed above, the collection of historical cell-site records is the subject of extensive statutory regulation at both the federal and
state levels. The Jones concurrences are distinguishable on that basis alone.
If the Court must address the general viability of
the mosaic theory, the approach should be rejected. Under the “longer term” aggregation approach advocated
by Carpenter, the basic operation of Fourth Amendment doctrine would be thrown into doubt. The Court
would need to answer an extensive list of novel and
difficult questions to allow lower courts to implement
the short-term/long-term distinction. In effect, the
Court would need to create a parallel set of new Fourth
Amendment rules. This would be a remarkable challenge. The Court should not adopt the mosaic approach
without recognizing the complexities ahead.

 17
I elaborate on the difficult questions in my Mosaic
Theory article, see id. at 328-43, but here is an overview
of the main questions. First, which surveillance methods are covered by a mosaic approach? If the mosaic
theory applies to collection of historical cell-site records, does it also apply to collecting credit card records?
Bank records? Use of automatic license plate readers?
Use of public cameras installed by the government?
Collection of camera footage from cameras installed by
private actors? Monitoring of phone numbers dialed or
Internet Protocol addresses used? Visual surveillance?
Does the mosaic approach apply only to collecting location information, or does it apply to collecting any information – and if it is limited to location records, what
about records (such as credit card statements) that
permit plausible guesses about a person’s location?
Each technique, and many more, would need to be classified as included or excluded.
For each technique covered by the mosaic theory,
a host of questions would arise. The most obvious –
how long counts as “longer term” – is just the tip of
the iceberg. Other questions include: Is the time period
the same for different surveillance methods covered by
the mosaic approach? What if several people used the
device and no mosaic for any one individual can be constructed? And imagine the government uses two surveillance methods at the same time, such as GPS and
cell-site tracking together, to monitor a suspect. Does
that cut the window of allowed monitoring in half ?

 18
It gets even more complicated. Imagine the Court
tries to draw a bright-line rule. Let’s imagine, for the
sake of argument, that 21 days of surveillance always
triggers a mosaic search. Does switching surveillance
methods restart the clock? For example, can the police
do 20 days of GPS tracking, then 20 days of cell-site
monitoring, and then 20 days of Internet Protocol address monitoring? Or does each day of any surveillance
method count towards the 21-day total?
And is there a statute of limitations that resets the
clock? Imagine investigators collect 20 days of cell-site
records to avoid a search. Can they wait six months –
or a year or two – and then get another 20 days’ worth?
And what if they get records only covering four hours
per day, say, from 1pm to 5pm every day. Can they still
only get records for 21 individual days before triggering a search, or can they now get records for up to 126
days because they are only collecting records for part
of each day? And consider how the test works if there
are multiple investigations of the same suspect, such
as a federal investigation and a local investigation operating concurrently. Does the 21-day rule apply to all
investigators collectively, or does separation among investigations mean different 21-day clocks?
Next consider how to determine the reasonableness of a mosaic search and what remedies apply to
violations. Is a warrant required? If so, how can a mosaic warrant satisfy the particularity requirement
when it is inherently about aggregating surveillance

 19
from many places over time? If no warrant is required,
how much cause is required? Do all mosaic searches
require the same amount of cause, or do different mosaic techniques for time periods trigger different levels? What is the test for standing to challenge a mosaic
search? Does the exclusionary rule apply, and if so does
it apply to the entire surveillance that occurred or only
that which crossed the line into being a search?
Carpenter offers no answers to these questions.
“When the time comes to provide precise guidance to
law enforcement agents and lower courts,” Carpenter
writes, “this Court will have ample authority to do so.”
Petr. Br. at 30. Carpenter won’t even touch the simplest
question of how long is “longer term.” In some future
case, Carpenter says, the Court can “set bright-line durational limits.” Id. at 31. Just adopt the theory now, in
other words. You can confront the maddening implications of it later.
The Court should decline this invitation. Fourth
Amendment law requires certainty, see Belton, 453 U.S.
at 458, in part because the blunt instrument of the exclusionary rule may apply. Because the government
can lose its case if officers break the law, courts must
provide clear rules that enable investigators to steer
clear of violations. It is hard to see how courts can supply those clear answers under a mosaic theory. The
many questions it raises “would keep defendants and

 20
judges” – and for that matter, law professors – “guessing for years to come.” Riley, 134 S. Ct. at 2493.10
IV. WHETHER REASONABLE PEOPLE EXPECT
PRIVACY IN CELL-SITE RECORDS IS IRRELEVANT, AS THE THIRD PARTY DOCTRINE
IS ABOUT MANIFESTING SUBJECTIVE EXPECTATIONS OF PRIVACY.
The parties frame this case as being largely about
the reasonable expectation of privacy test. That test
has been the subject of extraordinary confusion. Both
courts and commentators have been unsure of what
makes an expectation of privacy “reasonable.” The uncertainty is understandable. The Court has mixed and
matched among four different frameworks that offer
different answers to what makes an expectation of
privacy reasonable. See Orin S. Kerr, Four Models of
Fourth Amendment Protection, 60 Stan. L. Rev. 503
(2007). I have called these four frameworks the probabilistic, private facts, positive law, and policy models.
See id. at 507-24. A close read of Carpenter’s brief reveals rotating use of each of the four models. See Petr.
Br. at 14-15 (probabilistic model); id. at 15-21 (policy

10

For this reason, if the Court concludes that the Fourth
Amendment governs government collection of historical cell-site
records, the same Fourth Amendment restriction should apply to
both short-term and long-term records collection. In that event,
the Court should also conclude that the intermediate standard of
18 U.S.C. § 2703(d) satisfies the Fourth Amendment. See Resp. Br.
at 50-55.

 21
model); id. at 21-23 (positive law model); id. at 24-29
(private facts model).
Fortunately, the Court can and should sidestep the
morass of the reasonable expectation of privacy test in
this case. Properly understood, this case is not about
the reasonable expectation of privacy test at all. Instead, like other cases concerning the third-party doctrine, it is actually about the subjective expectation of
privacy test. See Orin S. Kerr, Katz Has Only One Step:
The Irrelevance of Subjective Expectations, 82 U. Chi.
L. Rev. 113, 115, 127-29 (2015) (hereinafter Subjective
Expectations). From this perspective, Carpenter’s position attempts to eliminate the original intended role of
the subjective prong of the two-part Katz test that the
Court has adopted. See id. at 115. The Court should
reject Carpenter’s invitation to set the Fourth Amendment on that new and uncharted path.
Appreciating this point requires a close read of
Justice Harlan’s famous concurring opinion in Katz. At
the time of Katz, there were two distinct sets of Fourth
Amendment precedents on what is a search. One set
identified the spaces that could be the subject of
Fourth Amendment protection. See, e.g., Rios v. United
States, 364 U.S. 253 (1960) (taxi cab); United States v.
Jeffers, 342 U.S. 48, 51-52 (1951) (hotel room). But see
Hester v. United States, 265 U.S. 57, 59 (1924) (open
fields). The second set considered when a person
waived protection in an otherwise-protected space by
voluntarily revealing information to others such as undercover agents. See, e.g., Hoffa v. United States, 385
U.S. 293, 302 (1966); Lewis v. United States, 385 U.S.

 22
206, 210-11 (1966); Osborn v. United States, 385 U.S.
323, 325-27 (1966); Lopez v. United States, 373 U.S.
427, 438-39 (1963); On Lee v. United States, 343 U.S.
747, 749 (1952).
Justice Harlan’s two-part Katz test simply summarized that case law. The “twofold requirement” was
Justice Harlan’s “understanding of the rule that has
emerged from prior decisions” that explained how the
Fourth Amendment protected a “place.” Katz, 389 U.S.
at 360 (Harlan, J., concurring). One requirement was
that the government intrusion of a protected area be
“a place” about which “society is prepared to recognize”
an “expectation of privacy” as “reasonable.” Id. This
answered what spaces could be constitutionally protected. Conversation in homes and enclosed phone
booths could be protected but “conversations in the
open would not be protected.” Id. at 360-61.
Another requirement was that a person must
“have exhibited an actual (subjective) expectation of
privacy.” Id. at 361. Under this requirement, “objects,
activities, or statements that he exposes to the plain
view of outsiders” from inside a constitutionally protected space “are not protected because no intention to
keep them to himself has been exhibited.” Id. This accurately summarized the many cases holding that a
person had no Fourth Amendment protection against
the use of undercover agents even inside the home. See,
e.g., Hoffa, 385 U.S. at 302-03. The key was that an expectation of privacy had to be “exhibited,” that is,
demonstrated by an act. A person had to shield his
speech from others to have protection; one who shared

 23
information with others necessarily accepted the risk
they would reveal it. See id.
All of this matters because Carpenter is about the
second inquiry rather than the first. No one is characterizing the “place” where the cell-site records were obtained or stored. It is irrelevant whether “society is
prepared to recognize” an expectation of privacy in that
place as reasonable. This case instead is about cellphone users’ failure to exhibit a subjective expectation
of privacy against a phone company’s network connecting with the phone to route the user’s communications.
Using a phone that seeks a connection with local networks does not exhibit an expectation of privacy in the
fact of that connection. Properly understood, the reasonable expectation of privacy test has no bearing on
the issues raised in this case.
As I detailed in a recent article, some precedents
of this Court have confused this point. See Subjective
Expectations at 124-26. The “subjective” prong of the
Katz test was wrongly assumed to be truly subjective.
This confusion led the Court to move the disclosure
principle over to the objective part of the test under the
label of the so-called third-party doctrine. Id. The result is a doctrinal oddity. The two-part Katz test has
been reduced to one part, and the third-party doctrine
that was originally the subjective part of Katz is now a
special application of the objective part. Id. at 127-33.
No wonder so many scholars criticize the third-party
doctrine: It comes off as a strange application of the
reasonable expectation of privacy test. But that’s because it’s not an application of that test at all. The

 24
Court should retain the third-party doctrine but characterize it properly as the subjective test, recognizing
that it is distinct from the reasonable expectation of
privacy test.11
This background explains the inability of Carpenter and his amici to identify a clear rule to govern this
case. Carpenter seeks to do something truly new. He
wants to introduce a right to stop others (here, cell
phone companies) from disclosing to the government
information he has shared with them. Carpenter can’t
identify a clear rule because no Fourth Amendment
principle explains who can be stopped to talk about
what they know and what facts they can’t disclose. Any
doctrinal line would be made out of thin air.
That poses no problem for legislatures. Legislatures can enact a nondisclosure rule that prohibits
specific entities from disclosing specific kinds of information except pursuant to specific kinds of legal process. Congress has done exactly that in the Stored
Communications Act. See 18 U.S.C. § 2702, § 2703. But
it’s hard to introduce a Fourth Amendment nondisclosure rule to determine what is a search. No constitutional text, history, or caselaw offers guidance on what

11

In my Subjective Expectations article, I recommended
eliminating the subjective prong while retaining the third-party
doctrine. See id. at 133-34. I now think the more sensible approach
is to restore the original understanding of Katz with the thirdparty doctrine properly labeled the subjective prong and distinguished from the objective prong.

 25
such a private-party nondisclosure rule would look
like.
The Empirical Scholars look for answers in public
opinion polls and surveys. See Amicus Brief of Empirical Fourth Amendment Scholars at 2-10. They envision
the Katz test as protection against the unexpected:
Surprising disclosures to the government should require a warrant because they violate the expectations
of ordinary people. See id. at 10-16. That has never
been the law. “The concept of an interest in privacy
that society is prepared to recognize as reasonable is,
by its very nature, critically different from the mere
expectation, however well justified, that certain facts
will not come to the attention of the authorities.”
United States v. Jacobsen, 466 U.S. 109, 122 (1984) (emphasis added).
Nor should it become the law. The Constitution
already includes an institution, Congress, that is designed to reflect public opinion about disclosures of
information. It’s also hard to see why the Fourth
Amendment should protect against unexpected government action. And what disclosures are surprising to
people is likely based on what they read on the Internet, which is not exactly a reliable basis for constitutional decisionmaking. Finally, it would be difficult for
courts to implement a survey-based approach to what
disclosures of information should be a search. Public
opinion changes, and judges are not empiricists who
are trained to compare and critique new scholarly research. Empirical studies can be useful in some contexts within Fourth Amendment law. But they cannot

 26
provide the nondisclosure line-drawing that Carpenter
needs.
V.

THE ENACTMENT OF PRIVACY LEGISLATION DOES NOT GOVERN OR INFLUENCE
THE MEANING OF THE FOURTH AMENDMENT.

Carpenter also argues that the enactment of privacy legislation should lead the Court to hold that he
has Fourth Amendment rights in his cell-site records.
He makes this argument in two ways. First, he argues
that the existence of statutory privacy protections for
cell-site records reflect a societal expectation of privacy
in those records. Petr. Br. at 21-23. Second, he argues
that designations found in 47 U.S.C. § 222 create a proprietary interest that makes cell-site records the user’s
constitutional “papers” or “effects.” Petr. Br. at 32-35.
Both arguments should be rejected.
(a) Privacy Legislation Does Not Provide
Evidence of Reasonable Expectations of
Privacy.
The enactment of privacy legislation cannot bolster Carpenter’s case for constitutional protection for
two reasons. The first reason is implicit in the discussion above: Because this case is properly understood as
being about a failure to manifest a subjective expectation of privacy rather than reasonable expectations of
privacy, whatever reasonable expectations may exist
are not relevant. See Part IV, supra. The second reason

 27
is broader and more fundamental. Even if you assume
that this is a case about reasonable expectations of privacy, privacy legislation cannot provide a helpful guide
to assessing those expectations.
Here are the basic problems, drawing from a recent article that explores the issue in greater depth.
See Orin S. Kerr, The Effect of Legislation on Fourth
Amendment Protection, 115 Mich. L. Rev. 1117 (2017)
(hereinafter Effect of Legislation). First, privacy legislation does not signal Fourth Amendment values because it answers a very different set of questions. Any
legal regime regulating law enforcement access must
address three questions: What information is covered,
how much protection is given to what is protected, and
what remedies apply to violations. Because legislative
privacy laws typically answer all three questions differently from Fourth Amendment law, one can’t isolate
any specific answer and imagine it sheds light on the
constitutional framework when the two other answers
are different. Id. at 1140-44.
The Stored Communications Act offers an example. Although the federal statute protects cell-site records from disclosure, that protection offers no guidance
on whether the Fourth Amendment should do so. The
statute imposes no limit on the scope of records obtained; it uses a “specific and articulable facts” disclosure standard; and it rejects a statutory exclusionary
rule. Pet. App. 17a-18a. It’s not clear how the coverage
of the statute can inform a judgment about whether
the Fourth Amendment should apply given that the

 28
Fourth Amendment typically has a different scope, a
different standard, and different remedies. See Effect
of Legislation at 1140-44.
The same is true with state laws that adopt a warrant requirement for access to historical cell-site records. Even assuming the exclusionary rule applies to
violations of those statutes, the states only have the
constitutional authority to regulate state and local
governments. Under the Supremacy Clause, state laws
cannot regulate federal investigations. See, e.g., United
States v. Supreme Court of N.M., 839 F.3d 888 (10th
Cir. 2016). This makes it difficult to determine the lesson of a state law warrant requirement. Does it signal
a wish to impose strong protection against disclosure
of cell-site records, or does it signal only a wish to regulate collection by state and local officers? See Effect of
Legislation at 1144-47.
Further, if the adoption of warrant-based protections by a minority of states signals something about
the Fourth Amendment, what should we make of the
fact that a majority of states have not enacted such
protections? Does that mean most states see no privacy
implications in the disclosure of cell-site data? That
most states are fine with the federal standard? That
most states just haven’t yet reached the question?
Looking at state laws for insights about privacy expectations requires drawing lessons from very mixed
signals. Like mining legislative history for helpful comments, it is akin to “looking over a crowd and picking
out your friends.” Patricia M. Wald, Some Observations
on the Use of Legislative History in the 1981 Supreme

 29
Court Term, 68 Iowa L. Rev. 195, 214 (1983) (quoting
Judge Harold Leventhal). The better approach is to interpret the Fourth Amendment independently of statutory privacy protections. See Effect of Legislation at
1157-64.
(b) 47 U.S.C. § 222 Does Not Make Cell-Site
Records the User’s Fourth Amendment
“Papers”.
Carpenter’s argument that 47 U.S.C. § 222 makes
cell-site records the customer’s Fourth Amendment
“papers” should also be rejected. To be sure, the Fourth
Amendment concept of “papers” includes papers in
electronic form. See Katz, 389 U.S. at 362 (Harlan, J.,
concurring); United States v. Ackerman, 831 F.3d 1292,
1304 (10th Cir. 2016) (Gorsuch, J.). At the same time,
the Fourth Amendment only provides a right to persons in “their” papers, not in the papers of someone
else. U.S. Const. Amend. IV; Minnesota v. Carter, 525
U.S. 83, 92-97 (1998) (Scalia, J., concurring).
The cell-site records collected in this case were
the “papers” of the phone companies, not Carpenter.
Cell-site records are information that a company creates, and a company then decides to store on its computers, about how the company’s network was used.
Users have no legal right to access their cell-site records. The records belong to the companies not their users. Cf. id. (Scalia, J., concurring).
Carpenter’s argument to the contrary is based on
47 U.S.C. § 222, which imposes certain limitations on

 30
the disclosure of customer-related records. The problem is that rules regulating disclosure do not create a
property right in the regulated facts that belong to the
subject of the disclosure. Confidentiality is not property. The fact that information concerns someone does
not make that information his stuff. If the law limits
when Alice can tell the world about what she saw Bob
do, Alice’s recollection does not become Bob’s “papers”
or “effects.” Alice’s recollections belong to Alice, not
Bob. Cf. Eugene Volokh, Freedom of Speech and Information Privacy: The Troubling Implications of a Right
to Stop People from Speaking About You, 52 Stan. L.
Rev. 1049, 1063-81 (2000).
------------------------------------------------------------------

CONCLUSION
The judgment of the court of appeals should be affirmed.
Respectfully submitted,
ORIN S. KERR
Counsel of Record
2000 H Street, NW
Washington, DC 20052
(202) 994-4775
okerr@law.gwu.edu
October 2, 2017