Law enforcement is increasingly relying on a new tool when investigating crimes with no suspects: geofence warrants. Geofence warrants take advantage of geofence technology, which constructs a virtually bounded geographic area and identifies all users present within that area during a given time window. Google, the primary recipient of geofence warrants, has adopted a policy of objecting to any geofence request that is not a probable-cause warrant. So far, law enforcement has complied. This has caused courts and litigators to defer the question of whether, under Carpenter v. United States, a probable-cause warrant is necessary. Instead, these parties have located the legality of geofence warrants in less explored regions of the Fourth Amendment as applied to new technologies: probable-cause and particularity requirements, the few exceptions to those requirements, and the proper execution of a warrant.

This Note fills an analytical void by providing a comprehensive examination of these less explored regions. The Note first provides a technology primer, detailing the three steps involved in geofence warrants: the initial data dump, selective expansion, and unmasking. It then provides background on relevant Fourth Amendment law, explaining why the familiar “reasonable expectation of privacy” test has not yet proven dispositive in geofence-warrant litigation. After cataloguing burgeoning geofence litigation, the Note examines 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 answers the question of whether probable cause must be shown for each device included in a digital search, based in part on jurisprudence regarding checkpoints, area warrants, and searches of many people in a commercial location. The Note next examines the selective expansion and unmasking steps, arguing (1) that geofence warrants are unconstitutional general warrants because of the discretion given to law-enforcement officials in warrant execution; and (2) that these steps may impermissibly increase a warrant’s scope or constitute multiple searches under one warrant. The Note concludes by considering the broader implications of corporate policy shaping Fourth Amendment guardrails.

* Haley Amster is a law clerk at Covington & Burling LLP; J.D., Stanford Law School, 2021. Brett Diehl is a trial attorney at Federal Defenders of San Diego, Inc.; J.D., Stanford Law School, 2021.

Our deepest gratitude to Robert Weisberg for his encouragement, guidance, and insights. Thanks to Michael W. McConnell, Morgan N. Weiland, and the rest of the Constitutional Law Center for their support and guidance. Thanks to Jonathan Abel, David Sklansky, Jonathan Mayer, Orin Kerr, John Ellis, Rick Salgado, Todd Hinnen, Sierra Villaran, Laura Koenig, the participants of the Constitutional Law Center’s Works-in-Progress Workshop, and the students of the Legal Studies Workshop for their helpful comments and feedback throughout the drafting process. Thanks to editors and friends—Marty Berger, Marc Brunton, Julia Irwin, Jenny Jiao, Dan Kim, Matt Krantz, David Levin, Caro Sundermeyer, Daphne Thompson, Mitchell Wong, Jeffrey Xia, and Peggy Xu—who made this Note better with their insightful edits and commentary. And thanks to Tal Klement for immediately recognizing the many questions that geofence warrants raise. All views expressed are our own and do not reflect those of any current or former employers.