A bipartisan group of 12 senators has urged the Transportation Security Administration’s inspector general to investigate the agency’s use of facial recognition, saying it poses a significant threat to privacy and civil liberties.

“This technology will soon be in use at hundreds of major and mid-size airports without an independent evaluation of the technology’s precision or an audit of whether there are sufficient safeguards in place to protect passenger privacy,” the senators wrote.

“While the TSA claims facial recognition is optional, it is confusing and intimidating to opt out of TSA’s facial recognition scans, and our offices have received numerous anecdotal reports of Transportation Security Officers (TSOs) becoming belligerent when a traveler askes to opt out, or simply being unaware of that right,” the senators wrote. They added that in some airports the signage instructing flyers to step in front of a camera is prominently displayed while signs advising passengers of their right to opt out of face scan is “strategically placed in inconspicuous locations.”

To opt out of a face scan at an airport, a traveler need only say that they decline facial recognition. They can then proceed normally through security by presenting an identification document, such as a driver’s license or passport.