The Albuquerque Police Department is among numerous U.S. law enforcement agencies using highly secretive technology that can track the whereabouts of crime suspects by using signals emitted by their cellphones.

The Albuquerque Journal reports that documents obtained by the American Civil Liberties Union of New Mexico following a lawsuit against the city indicate Albuquerque police have had at least two of the tracking devices for the last six years.

Former Mayor Richard Berry’s administration withheld information about the trackers but Mayor Tim Keller’s administration recently released the documents to the ACLU, which agreed to dismiss the lawsuit.

Purchasing agreements with the FBI restrict information that police can release about the devices and their use.

Other cities where police use cellphone trackers include New York, Los Angeles, Chicago and Las Vegas.