A geofence (or "geo-fence") is a virtual perimeter around an actual area. A geofence warrant is a court order authorizing the government to collect digital location-history data from private companies (such as Google) for a particular geographic area over a particular time frame. This data will (presumably) identify (nearly) every person who carried a cellphone (whether walking or driving or cartwheeling) through or past the area during the designated time frame.
Geofence warrants raise a host of Fourth Amendment questions. Do they authorize "searches" in the Fourth Amendment sense? What kind of probable-cause showing is required to support them? What about particularity? Are all geofence warrants unconstitutional general warrants? Or can they be sufficiently narrowed to satisfy Fourth Amendment requirements?
Few courts have grappled with these questions to date. A Westlaw search of all state and federal databases for "geofence warrant" yields only five results: one state trial court order and four federal district court orders. The most recent of these is an order from a D. Kan. Magistrate Judge denying (without prejudice) the government's application for a geofence warrant targeting the area around a building where a federal crime allegedly occurred. In the Matter of the Search of Information that Is Stored at the Premises Controlled by Google, LLC, 21-mj-5064-ADM, 2021 WL 2401925 (D. Kan. June 4, 2021).
In the order, the magistrate judge notes that "it is easy for a geofence warrant, if cast too broadly, to cross the threshold into unconstitutionality because of a lack of probable cause and particularity, and overbreadth concerns under Fourth Amendment jurisprudence." The magistrate judge then explains how the affidavit submitted in support of this warrant failed on both the probable-cause and particularity fronts.
First, probable cause that a crime was committed at the location is not enough. The government must also show probable cause that evidence of the crime will be found in the location data sought. Second, particularity requirements apply to both the geographic area and the time frame. If the government seeks a full hour of data, for instance, it must explain why. In sum:
The court simply issues this opinion to provide fair notice that geofence warrant applications must sufficiently address the breadth of the proposed geofence and how it relates to the investigation. It is not enough to submit an affidavit stating that probable cause exists for a geofence warrant because, given broad cell phone usage, it is likely the criminal suspect had a cell phone. If this were the standard, a geofence warrant could issue in almost any criminal investigation where a suspect is unidentified. The Fourth Amendment requires more, particularly where the warrant implicates the privacy interests of individuals who have nothing to do with the alleged criminal activity.
Want to read more? Check out this recent Harvard Law Review Note on the subject, and this Electronic Frontier Foundation article. Ask whether a geofence warrant was used in your case; request related discovery; and consider moving to suppress the fruits of the warrant. As the EFF has warned, "[i]ndiscriminate searches like geofence warrants both put innocent people in the government’s crosshairs for no good reason and give law enforcement unlimited discretion that can be deployed arbitrarily and invidiously. But the Framers of the Constitution knew all too well about the dangers of overbroad warrants and they enacted the Fourth Amendment to outlaw them."