A bipartisan group of 12 senators on Wednesday sent the Department of Homeland Security’s (DHS) inspector general a letter expressing alarm over the widespread use of facial recognition technology at American airports without an audit of privacy protections or any third-party assessment of the technology’s accuracy.

The Transportation Security Administration (TSA), which is housed within DHS, will soon roll the technology out in small and mid-size airports, taking the total number of airports where it is deployed to 430 nationwide, the letter to Inspector General Joseph Cuffari said.

The senators questioned whether facial recognition technology is necessary at all, since TSA already uses devices known as CAT-1 scanners — which don’t take images of faces — to determine if passengers’ identification is fake.

“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. “TSA has not provided Congress with evidence that facial recognition technology is necessary to catch fraudulent documents, decrease wait times at security checkpoints, or stop terrorists from boarding airplanes.”

TSA officials had told lawmakers and the public that face scans are not required, the letter said, but agency leadership has more recently said that they plan to expand the technology’s use beyond security checkpoints. Citing comments the head of the TSA made at a conference last year, the letter said the agency plans to make face scans mandatory in the future.

“If that happens, this program could become one of the largest federal surveillance databases overnight without authorization from Congress,” the letter said.

The senators urged Cuffari to evaluate the effectiveness of the technology; whether it meaningfully speeds up screening; whether it has prevented those on no-fly lists from boarding planes; and to assess TSA’s biometric data collection, storage and deletion protocols.

The letter was signed by Sens. Jeff Merkley (D-OR); John Kennedy (R-LA); Ed Markey (D-MA); Ted Cruz (R-TX); Roger Marshall (R-KS); Steve Daines (R-MT); Ron Wyden (D-OR); Elizabeth Warren (D-MA); Chris Van Hollen (D-MD); Cynthia Lummis (R-WY); Bernie Sanders (I-VT); and Peter Welch (D-VT).