The U.S. Supreme Court today ruled that law enforcement’s use of warrants to collect large amounts of cellphone location data requires privacy protections under the Fourth Amendment.

The case centered on a bank robbery, where prosecutors relied in part on cellphone location data obtained from Google LLC through a “geofence warrant.” Unlike traditional warrants aimed at a specific suspect, geofence warrants allow law enforcement to collect information on all cellphone users who were near a crime scene.

The armed robber had fled with around $195,000 from a bank in Richmond, Virginia. Law enforcement used a geofence warrant to a track a man named Okello Chatrie. Chatrie had before opted in to Google’s location history feature, thereby tracking his location every few minutes.

The data showed that Chatrie had been near the bank, the Call Federal Credit Union in Midlothian, at the time of the crime. When police searched his home, they discovered about $100,000 in cash. After pleading guilty, Chatrie received 12 years in prison.

His lawyers later argued that the geofence warrant violated his Fourth Amendment rights, which protect Americans against unreasonable searches and seizures. By a 6-3 majority, the court agreed that collecting location data through geofence warrants constitutes a search under the Fourth Amendment, meaning it is subject to those constitutional protections.

“A cellphone user is not to be viewed as sharing private information with third parties — which then can be freely passed on to the government — just by doing the ordinary things cellphone users do,” said Justice Elena Kagan. She likened such searches as creating a “virtual panopticon” where all Americans are exposed to surveillance regardless of their status.

Prosecutors said that Chatrie had no expectation of privacy because he had voluntarily opted into Google’s location history.

The judges in the majority argued that people do not knowingly consent to sharing their movements with third parties or the government. “Google repeatedly prompts users to turn on the service, often warning that devices will not ‘work correctly’ otherwise, while not disclosing in that prompt how frequently users’ location information would be recorded, how precise it would be, or how it might be given to the government,” they wrote.

The broader concern is that tracking people’s movements can reveal an extraordinary amount of information about their private lives. If left unchecked, the government could monitor ordinary citizens while claiming to be investigating a criminal.

The case will now return to the Fourth Circuit Court of Appeals to address the remaining legal questions.

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