Customs and Border Protection has been testing a number of biometric programs, teaming up with several airlines in Atlanta, Boston, New York and Washington. It will cost up to $1 billion, raised from certain visa fee surcharges over the next decade.

Customs officials say the biometric system has also produced some successes in the pilot testing and has helped catch people who have entered the United States illegally and are traveling on fake documents. They noted that facial scans and fingerprints — unlike travel documents — cannot be forged or altered and therefore give agents an additional tool to ensure border security.

But Senators Edward J. Markey, Democrat of Massachusetts, and Mike Lee, Republican of Utah, expressed concerns about the report’s findings. In a letter to Kirstjen Nielsen, the homeland security secretary, the senators urged the department to delay rolling out the facial scans until it addressed the privacy and legal concerns identified in the report.

In 1996, Congress ordered the federal government to develop a tracking system for people who overstayed their entry visas. After the Sept. 11, 2001, attacks, an entry- and exit-tracking system was seen as a vital national security and counterterrorism tool. The 9/11 Commission recommended in 2004 that the newly-developed Department of Homeland Security complete a system “as soon as possible.” Congress has since passed seven separate laws requiring biometric entry-exit screening.

But for years, officials have struggled to put a biometric exit system in place because the technology to collect the data was slow to take hold. And many American airports, unlike those in Europe and elsewhere, do not have designated international terminals, leaving little space for additional scanning equipment.

The biometric system being tested by the Department of Homeland Security can be used either with a small portable hand-held device or a kiosk equipped with a camera.