SALT LAKE CITY — Sen. Mike Lee wants to stop the expansion of a Department of Homeland Security facial recognition scanning program at U.S. airports over concerns that it violates Americans' privacy.

The Utah Republican and Sen. Edward Markley, D-Mass., sent a letter to Homeland Security Secretary Kirstjen Nielson on Thursday saying Congress has never authorized biometric exit scanning for American citizens. They urged the agency to delay rolling out the facial scans until it addresses privacy and legal questions.

"American citizens should not have to choose between traveling internationally and ensuring the security of their personal data," the senators wrote.

Homeland Security requires passengers departing on international flights at nine U.S. airports to submit to a face scan so their faces can be compared to the department's biometric database to verify their identity. The department intends to expand the program to all the country's international airports in 2018.

Airports in Atlanta, Boston, Chicago, Las Vegas, Miami, New York City, Houston and Washington, D.C., use facial recognition scanning.

The Center on Privacy and Technology at Georgetown University Law Center concluded in a report released Thursday that Homeland Security improperly gathers data on Americans leaving on foreign flights. The technology also may frequently make mistakes.

The department should stop scanning Americans as they leave the country and prove that face scans are capable of identifying impostors without inconveniencing everyone else, the report said. The system erroneously flags as many as one in 25 passengers for further scrutiny.

That means thousands of travelers could be wrongfully denied boarding each day, Lee and Markley say.

Neither Congress nor Homeland Security has justified the need for the billion-dollar program, which, according to the report, is on "shaky" legal ground.

"The privacy concerns implicated by biometric exit are at least as troubling as the system's legal and technical problems," the report says.

Lee and Markley want Homeland Security to answer questions about accuracy and potential flaws in the technology, how the program will not unduly burden travelers, including certain races or gender, and how it is improving visa overstay travel fraud.

The senators say the department should advise American citizens that they can opt out of biometric scanning. Though Homeland Security stores the data for only 14 days, there doesn't appear to be safeguards to prevent it from spreading to third parties or other government agencies, they say.

The Georgetown report says Homeland Security has acknowledged that it is unable to determine whether the face scans' accuracy varied depending on travelers' characteristics. Innocent people may be pulled from the line at the boarding gate and be subjected to fingerprinting at higher rates because of their complexion or gender, according to the report.

The report also says visa overstay travel fraud could — in theory — be a problem worth solving.

Foreign nationals who want to stay in the country without detection after their visas expire could be arranging to have others leave in their place with fake credentials. But Homeland Security has only shown limited and anecdotal evidence of that, according to the report.