The Department of Homeland Security has detected in the Washington area what appears to be the unauthorized use of a controversial technology that allows for the surreptitious surveillance of people’s cellphones — though it has not been able to pinpoint who or what is causing it, the department revealed in a letter released Tuesday.

The technology, a cell-tower simulator commonly known as a StingRay, has been deployed for years by federal and local law enforcement to pinpoint suspects’ locations, though its unauthorized use in the Washington area raises fears that foreign adversaries might also be taking advantage of it to spy on U.S. citizens.

The simulators work by tricking cellphones nearby to register with them, rather than normal cell towers. Once the device finds the phone it is seeking, it can pinpoint the phone’s location. Some versions of the technology can also be used to eavesdrop on calls.

Sen. Ron Wyden (D-Ore.) had asked the DHS whether it had detected foreign governments using the devices in the national capital region and elsewhere. The department’s revelation came in response to his request, though it had not “validated or attributed such activity to specific entities or devices,” officials said. It also did not provide any details on what was detected, other than to say it was “activity” consistent with the cell-tower simulator devices.

The development was first reported by the Associated Press.

Privacy advocates have long raised concerns about the government’s use of the technology without a warrant, especially in criminal cases, and law enforcement in the United States has fought to keep secret how it works and when it is used. In one Florida case, prosecutors offered a robbery suspect a deal that would allow him to plead to a second-degree misdemeanor rather than show his defense attorneys the device they used.

The America Civil Liberties Union has identified 73 agencies in 25 states and the District of Columbia that own the devices, though the organization says that count is probably low, as “many agencies continue to shroud their purchase and use of stingrays in secrecy.”

Devlin Barrett contributed to this report.