A federal judge has shot down a widely used police tactic, ruling the blanket collection of cellphone location data is unconstitutional.

Police departments will sometimes use cellphone data to find out who was in the vicinity of a crime in an effort to locate possible suspects. With a geofence warrant, the police can find out how many cell phones were near the crime, and who those cell phones belong to.

While the practice has proven very effective, critics have accused police of violating the Fourth Amendment, since the warrant doesn’t target a single individual, but scoops up the location data of everyone in a certain area, the majority of whom are innocent. Since the Fourth Amendment protects against unreasonable search and seizure, critics and privacy proponents believed that protection should extend to a person’s phone and location data.

According to NBC News, Judge M. Hannah Lauck of the U.S. District Court for the Eastern District of Virginia agrees, ruling the practice unconstitutional.

The ruling is a major win for privacy advocates, although it remains to be seen if the government will appeal.