WASHINGTON (Reuters) - Four U.S. senators on Wednesday urged the U.S. Homeland Security Department (DHS) to disclose additional information about unusual cellular surveillance activity that has been detected around the nation’s capital.

The Washington Monument stands in Washington, D.C., U.S., September 11, 2017. REUTERS/Joshua Roberts

The senators - Ron Wyden and Ed Markey, both Democrats, and Republicans Rand Paul and Cory Gardner - said the Trump administration should make public additional details about possible surveillance using cellphone-site simulators around Washington.

“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 four said in a letter reviewed by Reuters.

Homeland Security did not immediately comment, but said in a March 26 letter to Wyden that DHS “has observed anomalous activity in the National Capital Region that appears to be consistent with International Mobile Subscriber Identity (IMSI) catchers. (DHS) has not validated or attributed such activity to specific entities or devices”.

The senators’ letter also said that DHS had briefed federal agencies in February about the issue but has not made details of the briefings public.

Federal Communications Commissioner Jessica Rosenworcel, a Democrat, on Tuesday said at a commission meeting the issue was serious and surveillance tools could potentially be in use “by foreign or criminal actors”.

She warned “these surveillance tools can transform cell phones 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 added “the security of our communications is at stake right here, right now in Washington and this agency owes the public more than silence”.

If accurate, she said, “someone needs to explain how foreign actors are transmitting over our airwaves without approval from this agency”.

This month, three senior U.S. House Democrats asked FCC chairman Ajit Pai to take immediate action to address what could be foreign governments’ “surveilling Americans in the nation’s capital”.

The FCC only allows the devices, which are commonly known as “stingrays,” to be used by U.S. law enforcement agencies.

The devices can trick suspects’ cell phones into revealing their locations.

They mimic cell phone towers in order to force cell phones in the area to transmit “pings” back to the devices, enabling law enforcement to track a suspect’s phone and pinpoint its location.