"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."
The letter from Senators Ron Wyden, Cory Gardner, Rand Paul and Edward Markey was addressed to the Department of Homeland Security’s NPPD Director Christopher Krebs.
A press release accompanying release of the letter explained that by tricking mobile devices and rerouting mobile communications, so-called stingrays and other cell phone surveillance devices can reveal the location of a cellphone, eavesdrop on calls or plant malware.
READ MORE: US National Security Agency Deleted Surveillance Data — Reports
The letter cited a February 6 Power Point presentation on "Mobile Threats" given to federal government employees that was unclassified but marked "For Official Use Only."
Earlier in April, the US Department of Homeland Security (DHS) has admitted the presence of devices in the US capital that could allow criminals or foreign governments to track and monitor the communications in Washington, DC.
The US agency reported that the use of the IMSI catchers could pose threat to national security, while Christopher Krebs, the DHS official leading the NPPD, said in a separate letter also released by The Hill that the use of such IMSI catchers "is unlawful and threatens the security of communications, resulting in safety, economic and privacy risks."