(CNN) —  

The Department of Homeland Security has confirmed that it has detected evidence of mobile snooping devices around Washington, DC.

The devices could be the work of foreign governments or entities, however, DHS hasn’t determined their origin, the agency said in a letter. At issue are what are known alternatively as Stingrays, IMSI catchers or cell-site simulators. The devices essentially act as fake cellphone towers, and as mobile devices connect to them, the devices are able to snoop on the traffic that goes through.

Responding to an inquiry from Democratic Sen. Ron Wyden of Oregon, DHS top infrastructure and cybersecurity official Christopher Krebs confirmed that DHS has detected activity consistent with such devices. Law enforcement in the US does use such techniques as well, though courts have been scrutinizing that.

DHS “has observed anomalous activity in the National Capital Region (NCR) that appears to be consistent with International Mobile Subscriber Identity (IMSI) catchers,” Krebs wrote. “NPPD has not validated or attributed such activity to specific entities or devices. This information was reported to our federal partners at the time it was observed.”

Krebs was responding to a question from Wyden as to whether DHS had evidence of foreign IMSI catchers operating in the DC area, but Krebs repeatedly declined to attribute the activity to any entity in particular.

He did say the use of Stingrays “by malicious actors to track and monitor cellular users is unlawful and threatens the security of communications, resulting in safety, economic and privacy risks,” but he said DHS would need more resources to develop an ongoing ability to detect the use of the devices.

CNN’s Sophie Tatum contributed to this report.