A powerful bipartisan pair of senators is pressing the Obama administration over the development of spy tools that mimic cell towers in order to track people’s cellphones.

Senate Judiciary Committee Chairman Chick Grassley (R-Iowa) and ranking member Patrick Leahy Patrick Joseph LeahyBattle over timing complicates Democratic shutdown strategy Hillicon Valley: Russia 'amplifying' concerns around mail-in voting to undermine election | Facebook and Twitter take steps to limit Trump remarks on voting | Facebook to block political ads ahead of election Top Democrats press Trump to sanction Russian individuals over 2020 election interference efforts MORE (D-Vt.) sent a letter to Attorney General Eric Holder noting that they have “additional concerns” about the tools, which were reportedly developed by the CIA. The letter was sent last week but revealed to the public on Monday.

ADVERTISEMENT

The tools, known as “StingRays” or “IMSI-catchers,” imitate cell towers in order to track people’s location and identifying information through their phone. In addition to local police forces, the U.S. Marshals Service reportedly attaches the devices to planes depart from major airports, seemingly give it the power to spy on thousands of people down below.

The CIA’s role in the development of the technology was unknown until a Wall Street Journal story earlier this month. The spy agency’s role in the program is unusual, given that it is largely banned from domestic spying.

The two senators previously raised their concerns to Holder and Homeland Security Secretary Jeh Johnson last year, but they have not yet received a response.

The department’s “failure to answer these questions has heightened our concerns,” the pair of lawmakers wrote.

Now, they want details about the Justice Department’s rules for using the devices and whether it has ever tested out devices in the U.S. on behalf of intelligence agencies.