The Volokh Conspiracy

Mostly law professors | Sometimes contrarian | Often libertarian | Always independent

Court Upholds "Geofence" Warrant for Information on Which Phones Were Near a Crime


From In the Matter of Search of Information Stored at Google, 2021 WL 6111531, decided last week by Magistrate Judge Michael Harvey (D.D.C.); I unfortunately couldn't find a free version available online (the case is sealed on PACER), but here's an excerpt:

Imagine a homicide in an alley caught on a nearby surveillance camera. The video is clear enough to see the attack, but too grainy to identify who did it. It is obvious from the video, however, that the perpetrator is checking his cell phone as he walks out of the alley. Having determined the location and time of the killing from the video, would it be lawful for the police to obtain a warrant leveraging the tracking capability of mobile devices to identify whose cell phone was in the area of the crime when it occurred? On October 6, 2021, the Court was presented with a warrant that asked that question, albeit not in a homicide case. {The Court granted the government's request to seal the warrant application because the criminal investigation is not public and revealing the existence of the warrant could adversely impact the government's investigation, including by causing the subjects of the investigation to flee or destroy evidence. Accordingly, the public version of this memorandum opinion will not disclose facts that may identify the government's investigation or the targets of it.}

Commonly referred to as a "geofence" warrant, the government's application asked the Court to direct technology company Google to identify, through a multi-step process, the cell phone users that crossed into a defined geographic area around where the criminal activity under investigation occurred.

Though geofence warrants raise a number of important constitutional questions, there is not much federal caselaw discussing their legality. As of the date of this decision, the Court could identify only four federal [District Court] decisions, three of which denied the geofence warrant under consideration. That seeming reluctance to grant such warrants does not appear to have slowed their acceptance by other courts and their use by law enforcement. According to a recent report, Google received over 11,554 geofence warrants in 2020, up from 982 in 2018; as of August 2021, they comprised nearly a quarter of all warrants served on Google. Each of those warrants was authorized by a judge. So, it would appear that many more geofence warrants are being granted by courts than denied….

Soon after the advent of smart phones with the capability to track the location of their users, law enforcement sought warrants, or other legal process, to obtain the Global Positioning System ("GPS") data such cell phones collect to track their users who were known to be engaging in criminal activity. Often referred to as "GPS warrants," such warrants are, at this point, routine. They are frequently authorized, for example, to permit law enforcement to track a known or suspected drug dealer's cell phone to assist in locating the dealer's points of sale, drug stash houses, suppliers or co-conspirators.

The warrant before the Court is different. It is what has been termed a "reverse-location" warrant: the perpetrator of the crime being unknown to law enforcement, the warrant identifies the geographic location where criminal activity happened and seeks to identify cell phone users at that location when the crime occurred. The "geofence" is the boundary of the area where the criminal activity occurred, and is drawn by the government using geolocation coordinates on a map attached to the warrant…. The geofence is also bound by a time window dictated by when the crime is believed to have occurred (for example, between 1:00 p.m. and 1:15 p.m. on February 15, 2021)….

The cell phone location information Google collects [from Google OS phones and other phones running Google apps] is usually quite accurate—to within 20 meters, according to Google…. However, the location information Google collects is not perfectly precise, and includes a margin of error. Although the margin of error for each device depends on the quantity and quality of the location information the device transmitted to Google, Google says it aims to accurately capture the location of "at least 68% of users." Simply put, when Google searches its servers for the devices within a defined location boundary, a device that is outside of the boundaries of the geofence may be listed as within the boundaries of the fence due to imprecision in calculating the device's exact location….

The final piece of the puzzle is connecting the location data Google collects back to a particular user. According to the warrant affidavit, Google does this by collecting a phone user's information when the user registers for a Google account. In the registration process, the user can provide Google with their name, physical address, email address, and bank information, among other identifying information. The account registration process is critical because many of Google's key application and features "are accessible only to users who have signed into their Google accounts" ….

To avoid exposing the details of an ongoing criminal investigation, the Court will only generally describe the government's geofence warrant application. The geofence data sought in this case covers a shipping center, where the government alleges federal crimes were committed…. To illustrate the geographic area where it seeks Google's location data, the government has drawn a triangle on a satellite map included as an attachment to the application…. [The area of the triangle appears to be] up to 875 square meters….

The government seeks a total of 185 minutes of geofence data for the geofence area. The 185 minutes are split into segments ranging from 2 to 27 minutes on 8 specified days over a roughly five-and-a-half month period, corresponding to the criminal activity under investigation…. The government can be relatively precise about the data it requests because it has obtained (via subpoena) CCTV footage from inside the shipping center showing the criminal activity as it occurs. The government represents that the footage shows suspects using cell phones—and in some cases more than one phone—when they engaged in that activity….

The government proposed a multi-step process for obtaining the geofence data from Google….

  1. Using Location History data, Google will identify those devices that it calculated were within the [geofence area] during the course of the time periods laid forth in [the warrant].
  2. For each device: Google will provide an anonymized identifier that Google creates and assigns to device for purposes of responding to this search warrant; Google will also provide each device's location coordinates along with the associated timestamp(s), margin(s) of error for the coordinates (i.e., "maps display radius"), and source(s) from which the location data was derived (e.g., GPS, Wi-Fi, Bluetooth), if available. Google will not, in this step, provide the Google account identifiers (e.g., associated with the devices or basic subscriber information for those accounts to the government.
  3. The government will then review this list to identify devices, if any, that it can determine are not likely to be relevant to the investigation (for example, devices moving through the Target Location(s) in a manner inconsistent with the facts of the underlying case).
  4. The government must then, in additional legal process to the Court, identify the devices appearing on the list produced by Google for which the government seeks the Google account identifier and basic subscriber information.
  5. In response to this additional legal process, the Court may then order Google to disclose to the government the Google account identifier associated with the devices identified by the government to the Court, along with basic subscriber information for those accounts….

The court then analyzes the legal issues in detail, and concludes:

Given that most of us have our cell phones by our side most of the time, their power to constantly monitor their own location has essentially turned them into highly-accurate, personal tracking devices, identifying our location wherever we are in the world to within 20 meters. For better or worse, this feature that makes mobile devices useful to us—that gives us directions when we are lost and helps us find our phones when they go missing—leaves a digital trail behind us, the disclosure of which many would consider an invasion of privacy.

Unsurprisingly, our phones' ability to trace our steps has also proven of great value to law enforcement when trying to determine who was at a particular location when criminal activity occurred. It is that tension between privacy concerns and legitimate law enforcement interests the Fourth Amendment seeks to address, and which warrants careful consideration of constitutional requirements when assessing the lawfulness of geofence applications. These competing interests are assessed in recognition of the Fourth Amendment's prohibition of only "unreasonable" searches and seizures.

Applying those Fourth Amendment principles here, the government has established probable cause that criminal activity, which it has identified, occurred within the proposed geofences, and that evidence related to that activity, which it has specified, will be found within them. Further, it has carefully limited the scope of the geofences to the approximate location where the criminal activity occurred and timeframes during which it took place. The fact that the geofences may reveal the location information of non-suspects does not render the government's warrant unreasonable given that the geofences and the … protocol have been crafted to minimize privacy concerns to the greatest degree possible while also preserving "the fundamental public interest in implementing the criminal law." …