Against Geofences.

• Author: Amster, Haley

Table of Contents Introduction I. The Technology Behind a Geofence Request A. The Sensor Vault B. Warrant Execution 1. Initial data dump 2. Selective expansion 3. Unmasking II. Geofences and the Fourth Amendment A. Is a Geofence a Fourth Amendment "Search"? B. Probable Cause, Particularity, and Warrant Execution III. How Courts Are Handling Geofence Warrants A. Northern District of Illinois Magistrate Opinions 1. Pharmaceutical sale investigation: first denial 2. Pharmaceutical sale investigation: second denial 3. Pharmaceutical sale investigation: third denial 4. Arson investigation B. District of Kansas Magistrate Opinion C. Ongoing State and Federal Litigation D. Preliminary Takeaways from the Early Litigation IV. Constitutionality of the Initial Data Dump A. Probable Cause 1. Geofences as Ybarra searches 2. Geofences as checkpoints 3. Geofences as area warrants 4. Takeaways B. Issues with the Particularity Requirement V. Constitutionality of Selective Expansion and Unmasking A. Geofences as General Warrants B. Selective Expansions as Increases in Scope C. Multiple Searches VI. Corporate Policy and Fourth Amendment Protections A. Absence of Legislation B. Corporate Constitutional Policy Conclusion Introduction *

Suppose a law-enforcement officer investigating a hit-and-run sets up a checkpoint near the site of the incident. The investigating officer stops each passerby and examines their cell phone location history to determine if they were present at the crime scene. This officer would be in violation of the Fourth Amendment for employing a checkpoint in the "ordinary enterprise of investigating" a crime. (1) Now suppose that officer obtains a warrant compelling Google to do the same thing--digitally. Different result? (2)

Since roughly 2016, law enforcement has used geofence warrants to help revive criminal investigations gone cold. (3) These warrants have become increasingly common, (4) and there are even indications that a warrantauthorized geofence was used to investigate the January 6, 2021 attempted insurrection at the U.S. Capitol. (5)

Geofence warrants "work in reverse" from traditional search warrants. (6) Instead of law enforcement requesting that a third-party provider produce the location history of a particular suspect's device, geofence warrants proceed first by giving investigators access to data for all cellular devices that were present near a crime scene around the time when the crime occurred. Through a series of iterative steps between the provider and law enforcement--without the further involvement of a magistrate judge--the provider produces additional location data with the goal of (l) helping law enforcement figure out which devices could have been those of the perpetrators; and (2) ultimately revealing the identities of the suspects.

Such sweeping searches can unearth the location history of a startling number of users. One 2019 geofence warrant authorized a geofence covering a total of 29,387 square meters (or 7.4 acres--about the size of five and a half American football fields) over a period of nine hours. (7) In response, the provider returned to law enforcement the location data of 1,494 cell phones. (8)

So far, Google has been the primary recipient of geofence warrants. This is in large part due to Google's location-history database, the Sensor Vault. Google uses the Sensor Vault to target advertisements, determine when stores are busy, help users track their movements, and provide traffic estimates. (9) But law-enforcement officials now also use the SensorVault for criminal investigations. In response to increasing government requests for information, Google has crafted a three-step, self-directed process for law-enforcement officials trying to obtain user data. As Google explained in a 2020 court filing, it has "instituted a policy of objecting to any warrant that fail[s] to include" its mandated tailoring process. (10)

In recent years, the number of Sensor Vault-directed geofence warrants has grown rapidly. According to data released by Google, geofence warrants "recently constituted] more than 25% of all [U.S.] warrants" received by the company. (11) Google disclosed that it received 982 geofence-warrant requests in 2018. (12) This figure, Google explained in a court document, represented "over a 1,500% increase in the number of geofence requests ... [as] compared to 2017." (13) In 2019, the number of geofence warrants received by Google increased by a further 755% over the previous year to 8,396. (14) In 2020, the last year for which specific statistics are publicly available at the time of writing, Google received 11,554 geofence warrants. (15) California law enforcement represents the most frequent geofence-warrant requester, having submitted 3,655 of the 20,932 requests logged by Google over the three-year period. (16) Texas law enforcement came in second with 1,825 geofence warrants submitted to Google. (17) By contrast, federal law enforcement submitted only 928 requests from 2018 to 2020. (18)

As geofences become more well-known, at least one crime victim's family has specifically urged investigators to request a geofence warrant. (19) The Department of Justice's Computer Crimes and Intellectual Property Section has held discussions with Google about geofences and, in at least one instance, provided a boilerplate geofence-warrant request form to an FBI agent. (20) Hawk Analytics, which frequently assists law-enforcement investigations across the country, (21) hosted a webinar for law enforcement called "Working with Google Geofence Reverse Location Search Records" and previously offered an online tool allowing investigators to obtain a "Google geofence warrant in a few 'clicks.'" (22) Reports of wrongful arrests due to geofence warrants have already emerged. (23)

Courts and legislatures have paid little attention to how the Fourth Amendment applies to geofence warrants. (24) This is largely due to the novelty of the tool: As of this writing, most litigation has been ex parte, only five magistrate opinions considering the issue have been unsealed, and some of the First state and federal challenges by criminal defendants are underway. (25) But the lack of attention may also be due to Google's unique role. Since the Supreme Court's landmark decision in Carpenter v. United States--holding that the production of seven days' worth of cell phone location information constitutes a Fourth Amendment search requiring a warrant (26)--litigation and scholarship have focused on whether non-Carpenter technologies also lead to Fourth Amendment searches. (27) For geofences specifically, however, Google's policy of objecting to any request not derived from a probable-cause warrant has deferred the familiar "is this a Fourth Amendment search" question. (28) Questions surrounding geofence warrants' legality thus occupy less explored regions at the intersection of new technology and the Fourth Amendment: probable cause, particularity, and proper warrant execution.

This Note fills an analytical void by providing a comprehensive examination of these underexplored Fourth Amendment warrant requirements. It proceeds in six parts. Part I is a technology primer, detailing the three steps involved in geofence warrants: the initial data dump, selective expansion, and unmasking. Part II provides a background of relevant Fourth Amendment doctrine, including a discussion of how Carpenter intersects with geofence warrants. Part III catalogs burgeoning geofence litigation, with a special focus on the first few federal magistrate opinions on the issue. Part IV considers the initial data dump, identifying the difficulty of meeting probable cause and particularity requirements due to the inherent breadth of the search. Here the Note analogizes to the search of many people located at the scene of a crime in Ybarra v. Illinois, (29) the use of digital checkpoints, and the use of area warrants. It then explores the difficulty of tailoring by (1) examining digital searches of multi-occupancy buildings; (2) surveying scholarship and litigation regarding tower dumps; and (3) suggesting particularized search protocols that could meet constitutional requirements. Part V examines the selective expansion and unmasking steps, arguing that geofence warrants are unconstitutional general warrants because of the discretion given to law-enforcement officials in warrant execution. Part V also argues that the selective-expansion and unmasking steps may impermissibly increase a warrant's scope or constitute multiple searches under one warrant. Finally, Part VI considers the broader implications of corporate policy driving Fourth Amendment guardrails.

1. The Technology Behind a Geofence Request

   A geofence warrant compels Google to produce data from its SensorVault location-history database. (30) Under Google's threat of noncompliance, most geofence warrants proceed in three steps: the initial data dump, selective expansion, and unmasking. This Part first explains the SensorVault and then elaborates on each of the three execution steps, drawing on unsealed search warrants from federal and state investigations as examples.

   1. The Sensor Vault

      Google's SensorVault is a prodigious pool of consumer location information, pioneered in part to target advertisements but now routinely used by law enforcement for geofence warrants. (31) Cell-service providers and other corporations also collect cell-site location information for various purposes. (32) Yet the SensorVault and linked internal Google databases are more expansive, storing user location information generated from "search queries," "users' IP addresses, device sensors," and "device signals including GPS, information cellular networks provide to a device, information from nearby Wi-Fi networks, and information from nearby Bluetooth devices." (33) Multiple inputs can be combined to estimate a user's location "to a high degree of precision." (34) Google...