The Bureau of Alcohol, Tobacco, Firearms and Explosives canceled its contract for a surveillance tool that enables warrantless tracking of mobile devices after lawmakers, a prosecutor and a judge raised concerns about the legality of the tool in criminal investigations.

ATF, the federal agency responsible for enforcing the nation’s gun laws, told The Associated Press that it discontinued what it called a “pilot” program using a tool called Webloc after Rep. Michael Cloud, a Republican from Texas, and Sen. Ron Wyden, a Democrat from Oregon, expressed reservations about the agency’s use of bulk commercial location data.

Webloc, which is made by a vendor called Penlink, sources data from consumer apps and advertising networks, which collect the location of mobile devices from consumers who download apps or browse the web. Such data is sometimes called “ad tech” and has been controversial in criminal law enforcement as it allows agencies to bypass warrant requirements to identify the mobile devices present in certain areas at specific times.

The U.S. Supreme Court ruled in 2018 that police needed a warrant to obtain historic movement data from cellphone companies on a criminal suspect. But it has never addressed the growing practice of commercially acquired data.

Other users of Webloc include the U.S. military and U.S. Immigration and Customs Enforcement but also local law enforcement agencies such as police in places like Elk Grove, Calif. and Durham, N.C. The technology has also expanded around the world, with the national police in El Salvador and Hungarian intelligence agencies as customers, according to a report from earlier this year from Citizen Lab, a group of researchers at the University of Toronto who investigate digital threats to civil society.