A bipartisan group of US lawmakers are pressing the Department of Homeland Security to release what they say are unclassified records related to the potential foreign government use of mobile phone surveillance devices in the nation's capital.
"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," four US senators, led by Democratic Sen Ron Wyden, wrote in a letter to a senior DHS official on Wednesday.
Sens Cory Gardner and Edward Markey, who are Democrats, and Sen Rand Paul, a Republican, cosigned the letter.
The lawmakers are asking DHS to release a PowerPoint presentation they say contains "additional details about the use of rogue IMSI catchers in the US." The presentation is marked For Official Use Only (FOUO), a loosely defined Defence Department classification applied to documents that, while unclassified, may be deemed inappropriate for public release.
The mobile phone surveillance devices are known by a variety of names, including "IMSI catcher" or "cell-site simulator." However, they're more widely known as the Stingray, a brand name of a cell-site simulator manufactured by a US defence contractor.
Cell-site simulators operate by mimicking base transceiver stations - popularly known as "cell towers" - and can be used to pinpoint the precise location of a suspect's mobile phone once identified by his or her unique international mobile subscriber identity (IMSI) number. The devices are known to be disruptive, jamming nearby phones and sucking up vast amounts of metadata from innocent bystanders.
In March, DHS acknowledged publicly that it had detected signs of unauthorised use of Stingray-like devices in the Washington area in response to an inquiry from Sen. Wyden last year. The agency said "anomalous activity" consistent with their use had been detected and that that activity may be caused by foreign actors seeking to track mobile phones and potentially intercept calls and messages.
Cell-site simulators are widely used by US authorities, including the FBI, Secret Service, and US Marshals. They are controversial, in part, due to the level of secrecy surrounding them. The government has argued in the past that releasing any information about the devices risks compromising national security investigations, although the devices are frequently used by state and local agencies to track suspects accused of lesser crimes.