The DHS admission came in a March 26 response to a November request from Oregon Sen. Ron Wyden (D), however the agency did not say how many devices were detected or where they found them.

The agency's response, obtained by The Associated Press from Wyden's office, suggests little has been done about such equipment, known popularly as Stingrays after a brand common among U.S. police departments. The Federal Communications Commission, which regulates the nation's airwaves, formed a task force on the subject four years ago, but it never produced a report and no longer meets regularly. -AP

American intelligence and law enforcement agencies use similar  eavesdropping equipment in the field, which can cost anywhere between $1,000 to around $200,000. The devices are typically the size of a briefcase but can be as small as a cell phone. Police use Stingrays to track down and implicate perpetrators of mainly domestic crimes.

The devices can be mounted in vehicles, drones, helicopters, and airplanes, allowing police to gain highly specific information on the location of any particular phone, down to a particular apartment complex or hotel room.

The Stingray units operate by tricking a cellular device into locking onto them instead of a legitimate cell tower - revealing the exact location of a particular phone. As AP notes, more sophisticated versions can eavesdrop on calls by forcing phones to step down to the older, unencrypted 2G wireless channel. Other Stingray devices can plant malware on a phone.