A Department of Homeland Security program discovered evidence of the surveillance devices, called IMSI catchers, as part of federal testing last year, according to a letter from DHS to Sen. Ron Wyden (D-Ore.) on May 22. The letter didn’t specify what entity operated the devices and left open the possibility that there could be alternative explanations for the suspicious cellular signals collected by the federal testing program last year.

The discovery bolsters years of independent research suggesting that foreign intelligence agencies use sophisticated interception technology to spy on officials working within the hub of federal power in the nation’s capital. Experts in surveillance technology say that IMSI catchers – sometimes known by one popular brand name, Stingrays – are a standard part of the toolkit for many foreign intelligence services, including for such geopolitical rivals as Russia and China…

“This admission from DHS bolsters my concern about stingrays and other spying devices being used to spy on Americans’ phones,” Wyden said in a statement on Thursday. “Given the reports of rogue spying devices being identified near the White House and other government facilities, I fear that foreign intelligence services could target the president and other senior officials.”