A group of four senators is asking the Department of Homeland Security to turn over more information about the possible use of devices that allow for surveillance of cellphone activity around Washington, D.C., according to a report.

Sens. Ron Wyden, D-Ore., Ed Markey, D-Mass., Rand Paul, R-Ky., and Cory Gardner, R-Colo., said the administration should publicly disclose the details of whether cell-site simulators, known as a Stingray, were used to conduct surveillance in the D.C. metro area last year.

“The American people have a legitimate interest in understanding the extent to which U.S. telephone networks are vulnerable to surveillance and are being actively exploited by hostile actors,” the senators wrote in their letter, according to Reuters.

The senators also noted that the Department of Homeland Security briefed other agencies about the possible use of cell-site simulators in February, but hasn’t released the details to the public.

The Department of Homeland Security told Wyden in a letter last month it “observed anomalous activity in the National Capital Region that appears to be consistent with International Mobile Subscriber Identity catchers.” The agency said it has not determined who is behind the use of the surveillance devices. It's also unknown how many cell-site simulators were found.

The Federal Communications Commission allows only state, local and federal law enforcement and public safety officials to use the devices, and the FBI is required to coordinate their use ahead of time.

During a FCC meeting Tuesday, Commissioner Jessica Rosenworcel, a Democrat, said the tools could possibly be used “by foreign or criminal actors.” She further explained that “these surveillance tools can transform cellphones into real-time tracking devices by mimicking legitimate cell towers and some may even have the technical capability to record the content of calls.”

Rosenworcel said “someone needs to explain how foreign actors are transmitting over our airwaves” without receiving approval from the FCC.