The US Supreme Court ruled on Monday that law enforcement agencies must obtain a valid warrant before conducting geofence searches, reaffirming that probable cause is required to subpoena cellphone location data.
The decision in Chatrie v. United States held that users' digital, on-device location data is private and that the Constitution places limits on the government's ability to access tracking data from tech companies. In the 6-3 ruling (PDF) written by Justice Elena Kagan, the Supreme Court found "an individual has a reasonable expectation of privacy in records about his cellphone's location" and that geofencing warrants "intrude on that constitutionally protected interest."
Privacy experts celebrated the decision. Alan Butler, executive director of the Electronic Privacy Information Center, said in a statement after the ruling: "EPIC applauds the Supreme Court's recognition that warrantless geofence searches are fundamentally incompatible with the Fourth Amendment's protections against unreasonable searches and seizures."
Geofence warrants and the Constitution
Geofencing is generally carried out by law enforcement officials when a case has no clear suspects. Police will draw a shape on a map around a crime scene, specify a time window to inquire about and serve a geofencing warrant to a tech giant requesting information about any connected devices present within those fences during that time.
After cross-referencing this information with any suspects or suspicious individuals, police can subpoena additional account details (email addresses, phone numbers, usernames and more) associated with devices within the geofence.
The Court ruling found that users are not necessarily voluntarily sharing their private information with a company like Google, meaning the third-party doctrine (PDF) -- a legal principle that states people have no expectation of privacy when it comes to data willingly shared with others -- does not apply.
As a result, the Fourth Amendment, which prohibits unreasonable searches and seizures, protects against the current iteration of geofencing warrants. Those warrants are far less stringent and have fewer legal guardrails than traditional search warrants.
Crucially, the Supreme Court did not outright ban law enforcement geofencing; rather, police must already have probable cause about a suspect and obtain a search warrant to use geofencing data more narrowly.
Critics point out that geofencing doesn't just ensnare potential suspects but harvests location data from everyone in the area. In cases where law enforcement draws a border around a large chunk of a map, an agency may be asking for information about millions of people at a time.
Geofencing has been referred to as a type of reverse searching, since authorities are using the location data to pin a crime to a suspect, rather than naturally discovering evidence that would suggest a suspect was in the area when the crime was committed.
Google, one of the most frequently subpoenaed tech companies, has taken its own steps toward protecting itself from geofencing warrants in recent years, shifting users' location data away from Sensorvault servers and moving it onto the users' devices. Nonetheless, police can still serve warrants to private individuals to subpoena the location information from their phones.
If this doesn't sound like the type of geofencing you're used to, that's because the term is colloquially used to describe several use cases for location-based technology. It can also refer to the technology behind smart home controls or Big Tech advertising practices. The Supreme Court's decision only applies to the warrants.
How this case came to be
The case elevated to the Supreme Court was based on Chatrie v. United States (PDF). The plaintiff in the case, Okello T. Chatrie, was arrested in 2019 after police connected him to a $195,000 bank robbery. Law enforcement requested data from Google, locating devices around the bank during the time of the robbery. The list of potential suspects narrowed from 19 people to just three, eventually leading to Chatrie's capture.
Chatrie's lawyer, Adam Unikowsky, argued (PDF) the police did not have probable cause to comb through his client's information, and instead turned to "instruments that allowed the government to search first and develop suspicions later."
Even if the data was found independently of a geofencing search, Unikowsky argued (PDF), a follow-up warrant would be unconstitutional because probable cause still wouldn't exist to "search the virtual private papers of every single person within the geofence merely because of their proximity to the crime."
It's unclear how the new precedent may affect the outcomes of past cases, including Chatrie's. Previous courts ruled that Chatrie's sentence would not change because the geofencing evidence was obtained in good faith, but the Supreme Court's ruling leaves the warrant's validity in question.
Chatrie's case returns to a lower appellate court, which will decide whether there was probable cause for a geofencing warrant. A representative of Chatrie's legal team did not immediately respond to a request for comment.
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