Facial recognition technology isn't science fiction -- and the federal government has already used it on thousands of Americans entering the United States through Dulles International Airport.

The U.S. Customs and Border Protection used a facial recognition system on around 5,000 people with U.S. ePassports at Dulles, in recent months, agency spokesperson Jennifer Evanitsky said in an e-mail statement.

The Dulles test, which ended in May according to Evanitsky, was part of a larger set of pilot programs designed to test the feasibility of using biometric tech at border checkpoints previously reported on by Motherboard when it began in March.

At the moment, the system is not being tested at any other airports, Evanitsky said. "Plans for future testing of the technology are still being discussed."

The news comes amidst a public debate over whether there should be more rules about the use of facial recognition technology. It currently faces no specific federal restrictions, although the government and private companies are already using deploying the technology. The FBI's Next Generation Identification database, which includes facial recognition capabilities, reached "full operational capacity" last year and tech giants like Facebook use it for things like identifying people in photos.

Some privacy advocates worry that pilot projects like the one at Dulles could be harbingers of more widespread deployment of facial recognition in public spaces down the line. "What starts out as a narrow program can often turn into something that's broad and has great civil liberties implication," said Harley Geiger, advocacy director and senior counsel at the Center for Democracy & Technology.

The Dulles test was used on travelers ages 18 and up who were in lines equipped with the technology, along with some who were randomly selected. It compared a digital copy of passport photos stored on a chip embedded into passports issued in recent years to a picture taken as the person went through customs with facial recognition software to help figure out if the person trying to enter the country actually matches up with their documents.

"The idea is to catch imposters," said Mark Cohn, the chief technology officer at Unisys Federal Systems, which provided some of the technology used in the test.  But a CBP officer, not the computer system, had the final say on if the person was able to enter the country during the test, Evanitsky said.

The narrow version of facial recognition tested out at Dulles doesn't raise as many privacy concerns as some others uses of the technology, said Alvaro Bedoya, the executive director of Georgetown Law's Center on Privacy & Technology, because it is using the technology to verify the identity of a person rather than identify them from a larger database. "But there are still privacy interests because whoever is collecting it could turn around and use it for some other purposes," he said.

The government took precautions to prevent those risks, according to a privacy impact statement released earlier this year. For instance, the government will only hold onto photos until the end of a 19-month pilot period, it said, unless the photos became part of a criminal investigation.

And the photos from the test won't be tied directly to an individual's identity, instead they will be stored in a database that tags them by the time and date the image was taken, according to the privacy impact statement. "The technology is a stand-alone system that does not communicate with any other CBP or DHS systems," Evanitsky told the Post. "CBP remains dedicated to protecting the privacy of all travelers."

There was information posted at the airport informing travelers about the trial, she said, and officers informed travelers about the test before using it on them. Travelers could not opt out of the testing if they were in a line using the equipment, according to Evanitsky.

Cohn said the United States is actually a bit behind the curve when it comes to using facial recognition technology at its borders. He believes it has the potential to speed up the customs process, perhaps even bolstering tourism, and points to other countries that already use similar set ups to automate parts of the immigration process.

"Years ago we integrated a system that used facial recognition and other biometric identifiers to detect counterfeit documents for the Chilean border police," he said, working to set up systems at the Santiago airport.

Travelers approaching some forms of these systems, which can look like picture kiosks, may not even understand they are using biometric data to verify a person's identity, Cohn said. "It's the sort of thing you might not even realize is happening — as long as you're not flagged as a non-match, you go through the process and have no reason to be aware of the automated check is going on."

This post has been updated with further details about the test from CBP.