Last week, the Department of Homeland Security briefed privacy advocates in San Francisco concerning its misguided plan to subject all international travelers to face scans at airports. Those at the briefing asked questions that should have been easy to answer: Why are airport face scans necessary? Is DHS’s technology accurate? When will DHS adopt public rules explaining what the program is, how it works, and how it will protect Americans’ privacy? But DHS had few responses—and that’s alarming.

Biometric exit is a massive—and massively expensive—solution in search of a problem.

Currently, DHS’s biometric exit program scans travelers at international departure gates at 11 U.S. airports and collects their biometric data (a fingerprint or face print). This includes both American citizens and foreign visitors, and DHS plans to expand the program to all of the country’s international airports within four years. In 2017, President Trump issued an executive order speeding up the implementation of biometrics at the border, and immigration reform legislation this year could include money to expand the current program.

Rather than bringing biometric exit to more airports, though, Congress should repeal the program altogether. It’s invasive, ineffective, and unnecessary. In short: It’s a billion-dollar boondoggle.

For one thing, biometric exit is a massive—and massively expensive—solution in search of a problem. In 2004, Congress told DHS to collect visitors’ biometric data at entry to detect criminals and terrorists before they set foot on American soil. But that rationale only works when visitors arrive. It’s less clear why DHS needs to do that as visitors leave. DHS officials say airport face scans are necessary to detect immigrants trying to cheat the visa overstay tracking system by using someone else’s ID when they leave. But DHS has failed to produce any evidence that that’s a serious problem. In fact, DHS’s own experts have questioned the program. According to the Government Accountability Office, “DHS reported in May 2012 that significant questions remained regarding … the additional value biometric air exit would provide compared with the current [nonbiometric] process, and … the overall value and cost of a biometric air exit capability.”

Furthermore, DHS hasn’t supplied any evidence that airport face scans will even accurately detect people attempting to leave the country using someone else’s credentials. If an impostor showed up at a departure gate bearing the passport of someone who looked a lot like him, the system might just give him a green light to get on the plane. When questioned about how often the system would let people through who aren’t supposed to be on a flight, a senior DHS official said in a September meeting that the department “wouldn’t know.” Asked again last week at the briefing for privacy advocates, officials declined to say whether DHS even tests its system’s ability to catch impostors, instead declaring that it’s “not fair to say what [DHS’s] vulnerabilities are.” If DHS does this testing, it should disclose the results so Congress, and the public, can assess whether the program is worthwhile. And if it doesn’t even know whether the system can catch imposters, then we’re talking about a bar hiring a billion-dollar bouncer without knowing whether he can spot a fake ID.

What’s more, it’s not clear whether its system will disproportionately reject people of a certain race or gender. A DHS official said in a September meeting with privacy advocates that the department doesn’t know because it only has “a small sample size so far.” Testing shows that face scans often struggle with race and gender bias. If a customs officer routinely stopped passengers for no reason other than because of the color of their skin he’d promptly be fired.

But when an agency uses a neutral-seeming algorithm to discriminate, it can be much harder to rein in.

DHS has said it will dispose of the data it collects within 14 days, restrict how that data is shared, and secure it from hackers. But this isn’t codified anywhere. In fact, DHS has failed to pass a single rule protecting Americans’ privacy and civil liberties when their faces are scanned. When pressed last week about the lack of rules, officials couldn’t provide any kind of timeline for when rules might come out. Without enforceable rules, Americans have no remedy if (or, let’s be honest, when) the government breaks its promises.

Given all the problems with biometric exit, some in Congress have started scrutinizing the program. Democratic Sen. Edward Markey and Republican Sen. Mike Lee have challenged DHS to explain the program’s legal authority, justification, and accuracy. The senators contend that Congress should not spend another dollar expanding DHS’s deeply flawed biometric exit system until they receive satisfactory answers to these basic questions. That’s common sense.

Trump wants Congress to push ahead anyway—and it might. House Judiciary Committee Chairman Bob Goodlatte’s immigration bill, supported by the Trump administration, would authorize the appropriation of another $1.25 billion over the next five years into biometric exit, in addition to the $1 billion Congress already appropriated. That would put the total for this program at more than what Congress is spending on the Cancer Moonshot, the National Cancer Institute’s initiative to accelerate cancer research. It’s hard to know exactly where Congress will land on immigration as it scrambles to negotiate before the next shutdown deadline Feb. 8. But it could include $1 billion or more for biometric exit.

Meanwhile, no one seems to have good answers to important questions about whether the program is justified, accurate, or clearly authorized. Not even DHS itself.