The US Secret Service has revealed plans for a test of facial recognition surveillance around the White House, with the goal of identifying “subjects of interest” who might pose a threat to the president. The document was published in late November, but the American Civil Liberties Union publicized its existence today. It describes a test that would compare closed circuit video footage of public White House spaces against a database of images — in this case, featuring employees who volunteered to be tracked.

The test was scheduled to begin on November 19th and to end on August 30th, 2019. While it’s running, film footage with a facial match will be saved, then confirmed by human evaluators and eventually deleted. The document acknowledges that running facial recognition technology on unaware visitors could be invasive, but it notes that the White House complex is already a “highly monitored area” and people can choose to avoid visiting. We don’t know whether the test is actually in operation, however. “For operational security purposes we do not comment on the means and methods of how we conduct our protective operations,” a spokesperson told The Verge.

It’s currently looking for employees, but could eventually be used for ‘subjects of interest’

Secret Service agents are already given photos to identify subjects of interest, who might be chosen based on social media posts, media reports, or “reports from concerned citizens.” If facial recognition is rolled out more broadly, it will serve as an automated version of that surveillance process.

The ACLU says that the current test seems appropriately narrow, but that it “crosses an important line by opening the door to the mass, suspicionless scrutiny of Americans on public sidewalks” — like the road outside the White House. (The program’s technology is supposed to analyze faces up to 20 yards from the camera.) “Face recognition is one of the most dangerous biometrics from a privacy standpoint because it can so easily be expanded and abused — including by being deployed on a mass scale without people’s knowledge or permission.”

The Department of Homeland Security (which the Secret Service operates within) already uses facial recognition to scan passengers on international and domestic flights, and it may expand the program to more US airports over the coming years. Meanwhile, some police departments have used a facial recognition tool built by Amazon, called Rekognition, to scan real-time camera footage against mugshots.

Facial recognition is a powerful tool that can be prone to error

These systems can make significant errors: one test falsely matched 28 members of Congress to mugshots last year. That’s not necessarily a problem, if the results are corrected by humans later. But if these tools become ubiquitous, it could lead to some people being mistaken (if only temporarily) for wanted criminals.

Legislators have sent Amazon multiple letters requesting information about Rekognition — the most recent, sent last week, cited “serious concerns that this type of product has significant accuracy issues, places disproportionate burdens on communities of color, and could stifle Americans’ willingness to exercise their First Amendment rights in public.”

As the ACLU writes, the Secret Service isn’t just tasked with protecting the White House. So if it incorporates facial recognition into its regular arsenal of defense, it could end up being deployed anywhere that a president or vice-president travels — which makes it an important issue even for people who won’t ever visit the White House.

Update 5:20PM ET: Added statement from Secret Service.