WASHINGTON (Reuters) - Two U.S. senators on Thursday urged federal authorities to halt the planned expansion of a $1 billion airport facial scanning program, saying the technology used to identify travelers on some flights departing from nine U.S. airports for international destinations may not be not accurate enough and raises privacy concerns.

Congress has approved the use of the program for non-U.S. citizens, but never expressly authorized its use for Americans. The Department of Homeland Security has said the system is needed to prevent travelers from leaving the country using someone else’s identity and to prevent visitors to the United States from overstaying their visas.

Senators Mike Lee, a Republican, and Edward Markey, a Democrat, in a letter to Department of Homeland Security Secretary Kirstjen Nielsen, raised concerns that too many travelers would be inconvenienced by faulty scan results and questioned why Americans are being subjected to the screens, known as biometric exit detection technology.

In the letter, they raised objections to expanding the program beyond the nine airports where it is already in use.

“We request that DHS stop the expansion of this program and provide Congress with its explicit statutory authority to use and expand a biometric exit program on U.S. citizens,” the senators wrote. “If there is no specific authorization, then we request an explanation for why DHS believes it has the authority to proceed”

They cited a report released Thursday by Georgetown University Law School’s Center on Privacy & Technology that found DHS is conducting the scans “without basic legal and technical safeguards - or any meaningful justification of its billion-dollar cost.”

Congress in 2016 authorized spending up to $1 billion over 10 years on the facial scans.

The U.S. Customs and Border Protection (CBP), on its website, says the program collects “facial images from all travelers from the United States” on a flight and uses the images to verify identities. It says the images are stored for no more than two weeks and says that “CBP is dedicated to protecting the privacy of all travelers.”

The government said Thursday that U.S. citizens may opt out of the facial screening and instead have a separate review of their ID documents.

A U.S. Customs and Border Protection spokeswoman said the government is working to establish the biometric exit program “in a way that’s most efficient and secure for the traveler and that is least disruptive for the travel industry.”

Airports using the system include Boston, Las Vegas, Miami, New York’s John F. Kennedy, Washington Dulles, both Houston airports, Chicago O’Hare and Atlanta.

The senators want DHS to provide data that the program will not unduly burden travelers. DHS said previously its goal is a 96 percent “true accept rate,” meaning the technology can positively identify 96 percent of the faces it scans.

The senators, however, said this meant “there would still be a false denial for one in 25 travelers. Further there is evidence that certain face scans exhibit different error rate depending on the race or gender of the person being scanned.”

DHS has said travelers who cannot be verified are escorted to another area where Customs and Border Patrol uses other methods to verify their identity.

The Georgetown Law report also noted that DHS has not established any rules governing the program.

“It’s as if DHS has hired a billion-dollar bouncer to check IDs but never checked how good he is at spotting a fake,” said Laura Moy, deputy director of the center and co-author of the report. “They also don’t know if he’s biased against certain groups of people.”

The senators said DHS also needed safeguards to ensure facial data is not shared with other U.S. agencies.