The Maryland Court of Special Appeals on Wednesday upheld a historic decision by a state trial court that the warrantless use of cell-site simulators, or Stingrays, violates the Fourth Amendment.

The trial court had suppressed evidence obtained by the warrantless use of a Stingray — the first time any court in the nation had done so.

Last April, a Baltimore police detective testified that the department has used Stingrays 4,300 times since 2007, usually without notifying judges or defendants.

The ruling has the potential to set a strong precedent about warrantless location tracking. “Police should now be on notice,” said Nate Wessler, a staff attorney with the ACLU’s Speech, Privacy, and Technology Project. “Accurately explain your surveillance activities to a judge and get a warrant, or risk your evidence being thrown out.”

Stingrays mimic cellphone towers, tricking nearby phones into connecting and revealing users’ locations. Stingrays sweep up data on every phone nearby — collecting information on dozens or potentially hundreds of people.

The case centers around the 2014 arrest of Kerron Andrews, a suspect in a shooting that injured three people. In order to locate him, police filed a “pen register” application, which is not a warrant, and does not require them to establish probable cause. A judge granted the application, which said that police would obtain the information from Andrews’ wireless service provider.

Instead, police used a high-tech Stingray called the “Hailstorm.” They located Andrews and found the murder weapon. However, they repeatedly failed to notify the judge about the change in tactics. Finally, during a hearing last June, the police department was forced to testify about the Hailstorm, leading the judge to accuse it of intentionally withholding information from the defense.

After the trial court threw out the Stingray evidence, the Maryland attorney general alarmed civil liberties groups by arguing that anyone who keeps their phone turned “on” is consenting to being tracked by police. The full ruling, which has not yet been issued, will presumably reject that argument.

During the oral argument before the appeals court in February, one of the judges called the police’s pen register application a “completely false document,” and “completely disingenuous.”

The Department of Justice issued guidelines in September requiring federal officers to apply for a warrant before using a Stingray. Those guidelines only applied to the seven agencies known to use them, not to state and local police. In 2014, the state of Maryland passed a law requiring a warrant for police to track an individual’s current or real-time location. The law only affects cases going forward, so it did not influence Andrew’s case.

Stingrays are also piquing the interest of lawmakers on Capitol Hill. Lawmakers held a hearing Tuesday on a bill that would require all police departments to get a warrant before using Stingrays. “Just because it’s easier in 2016 for law enforcement to track our location and learn intimate details about our lives, it doesn’t mean those details are somehow less worthy of Constitutional protection,” said House Oversight Committee Chairman Jason Chaffetz. “Get a warrant.”

In December, The Intercept published a secret catalogue of U.S. government surveillance equipment, including Stingrays. The advertisements for some items boast that they can spy on 10,000 people.

Top photo: A Stingray, manufactured by Harris Corp.