Geofence Warrants and the Fourth Amendment

A geofence warrant is, structurally, a Fourth Amendment anomaly. Rather than identifying a suspect and seeking authority to search the suspect's effects, the warrant identifies a location and a time window and seeks the identities of all persons whose mobile devices registered with a particular service — typically Google's Location History — within that spatial and temporal envelope. The investigative logic is inverted: the warrant does not constrain the search to evidence of a known target's crime but reverse-engineers a set of potential targets from the location data of all persons within the zone.

The constitutional question that this technique raises is not new in kind. The Fourth Amendment's particularity requirement — that warrants describe "the place to be searched, and the persons or things to be seized"¹ — has long been understood to forbid general warrants, of which the geofence warrant is, on one reading, a paradigmatic modern instance. The recent Supreme Court decision in Carpenter v. United States,² establishing that the acquisition of historical cell-site location information from a telecommunications carrier constitutes a Fourth Amendment search, supplied the doctrinal framework within which the geofence question must now be analyzed.

The Fourth Circuit en banc decision in Chatrie

The Fourth Circuit's recent en banc decision in United States v. Chatrie³ represents the most extensive appellate treatment of geofence warrants to date. The defendant was identified as a robbery suspect through a geofence warrant served on Google, which produced location data for several devices within a 150-meter radius of the crime scene during the relevant time window. The Fourth Circuit divided over whether the acquisition of the initial anonymized data tranche constituted a Fourth Amendment search at all — a majority concluding it did not, on the ground that the defendant retained no reasonable expectation of privacy in data he had voluntarily transmitted to Google through his use of the Location History service.

The dissents in Chatrie made the more constitutionally satisfying argument: that Carpenter's recognition of a privacy interest in CSLI applies a fortiori to the more precise location data generated by Google's Location History, and that the third-party doctrine cannot bear the weight that the majority placed upon it. The dissent also observed, correctly in our view, that the question whether the initial data acquisition is a "search" should not be determined by the analytical convenience of treating the warrant as a multi-step process in which the early steps escape scrutiny.

The Fifth Circuit's contrary path in Smith

The Fifth Circuit's decision in United States v. Smith⁴ took the opposite path, holding that geofence warrants violate the Fourth Amendment's particularity requirement because they amount to a "general, exploratory rummaging" through the location data of every person within the targeted zone. The court declined to suppress the evidence on good-faith grounds, but its constitutional analysis is significant: it treats the absence of particularity not as a curable defect of warrant drafting but as a categorical infirmity of the geofence technique itself.

A warrant that begins not with a suspect but with a polygon drawn on a map tests the particularity the Fourth Amendment was designed to require.

The doctrinal tension between Chatrie and Smith reflects, at bottom, a disagreement about the appropriate analytic frame. Chatrie treats the warrant as a multi-stage process in which the initial data acquisition is a discrete event to be evaluated under the third-party doctrine; Smith treats the warrant as a single constitutional event in which the particularity defect inheres in the technique. Neither framing fully accounts for the operational reality, but the Smith framing has the advantage of locating the constitutional inquiry where the framers would have expected it: in the relationship between the warrant's described scope and the government's articulable cause to search.

The Riley analogy and the categorical instinct

The Supreme Court's decision in Riley v. California,⁵ requiring a warrant for the search of cell phone data incident to arrest, supplied an early signal that the Court would not mechanically extend pre-digital doctrine to settings in which the practical consequences are categorically different. The geofence warrant invites a similar categorical analysis. The technique is not analogous to the canvassing of a neighborhood for witnesses, in which investigators speak with individuals who retain the ability to refuse cooperation; it is analogous to the seizure and inspection of every person's location records within a zone, conducted without their knowledge, on the prospective hope that one of them will turn out to be a suspect.

The path of the doctrine

The Supreme Court is likely to take up the geofence question in the near future. The circuit split is now well developed; the underlying technique is widely used by federal and state law enforcement; and the Court has shown, in Carpenter and Riley, a willingness to recalibrate Fourth Amendment doctrine for the digital environment. Our expectation, advanced tentatively, is that the Court will conclude that the acquisition of geofence data from a service like Google's Location History constitutes a search subject to the warrant requirement, and that the particularity analysis will require some demonstration of investigative tailoring that the existing practice typically fails to satisfy. For related commentary, see our piece on privacy torts in the era of persistent data and our analysis of government records and the modernization of transparency law.