Washington (CNN) A bipartisan group of four Senate privacy hawks are demanding the Department of Homeland Security publish more information about the evidence of mobile snooping devices in Washington and surrounding areas.

"The American people have a legitimate interest in understanding the extent to which US telephone networks are vulnerable to surveillance and are being actively exploited by hostile actors," Sens. Ron Wyden, D-Oregon, Cory Gardner, R-Colorado, Ed Markey, D-Massachusetts, and Rand Paul, R-Kentucky, wrote in a letter Wednesday to Christopher Krebs, the top infrastructure and cybersecurity official at the Department of Homeland Security.

In responding to an inquiry from Wyden, the department confirmed earlier this month it had detected activity consistent with devices known as Stingrays, IMSI catchers or cell-site simulators that essentially act as fake cellphone towers. As mobile devices connect to them, the devices are able to spy on the traffic that goes through, and can locate the user and whom the user is contacting.

Krebs did not attribute to the activity to any specific entity or device.

"These things have the capability of tracking. So, if you want to pick a person and say, let's see where they go and who they talk to during the day, that might give you just enough intelligence to make some decisions without even doing the eavesdropping," Kevin Murray, a counter espionage expert, told CNN in an interview.

Read More