The Department of Homeland Security has acknowledged "anomalous activity" in Washington, D.C. and elsewhere that could be foreign entities using devices that mimic cell phone towers in order to intercept calls.

The DHS National Protection and Programs Directorate (NPPD) "has not validated or attributed such activity to specific entities or devices." But as the Associated Press first reported this week, there is a concern these devices—known as International Mobile Subscriber Identity (IMSI) catchers, or Stingrays—could be in use by "foreign spies and criminals"

DHS discussed the devices in a letter to US Sen. Ron Wyden, who asked the agency in November whether foreign intelligence agencies and criminals could be using IMSI catchers to spy on the US government.

The agency did not answer that question directly, arguing that "the sources and methods that foreign governments use to conduct intelligence activities is generally classified." But if it were happening, that activity "would be unlawful and threaten the security of communication, resulting in safety, economic, and privacy risks," according to the agency. The use of IMSI catchers by foreign governments in particular "may threaten US national and economic security," DHS says.

In a separate letter, the agency added, "the malicious use of IMSI catchers is a real and growing risk."

Law enforcement can legally deploy Stingrays to catch criminals, though their use has been controversial.

The report isn't a total surprise. Security researchers and journalists have also noticed the use of IMSI catchers in the DC area. And although the surveillance tech is sold to law enforcement agencies, it can also be bought from black market vendors, or put together using off-the-shelf parts and open-source software.

Stopping the illegal use of the technology is another matter. DHS says it doesn't have the capability to specifically pinpoint the IMSI catchers. Doing so would require funding "to procure, deploy, operate, and maintain the capability, which includes the cost of hardware, software, and labor."