When the feds want to keep track of who is coming into the country, so-called biometrics may be the best way to do so. Unlike with documents, it’s very hard for a traveler to present a forged copy of a fingerprint or iris.

That’s why the U.S. Department of Homeland Security plans to vastly expand the amount of biometric data it collects at the borders. According to Passcode, a new program will ramp up a process to scan fingers and eyes in order to stop people entering and exiting the country on someone else’s passport.

The report says Homeland Security plans to introduce fingerprint scanning at 20 of the busiest airports by 2018 in an effort to reduce the number of “impostors” coming into the country. Right now, the technology is being used at JFK Airport in New York and at Dulles airport in Washington, D.C. The agency is also deploying iris and facial recognition software at a land crossing in Mesa, Calif.

“We truly believe that having biometrics on both exit and entry is going to be that next step in our transformative efforts to make the arrivals and departure processes the most efficient and secure it can be at the U.S. border for both U.S. citizens and our visitors,” a Homeland Security official told Passcode.

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While the expansion of biometric scanning offers the promise of better border security, it also poses privacy concerns, according to civil liberties groups cited in the report.

For instance, one risk is that the government may become tempted to expand the use of finger or iris scanning beyond borders, and begin tracking the movement of people within the country as well.

Large biometric databases could also become a target for hackers. Recall how Chinese spies broke into the computers of the Office of Personnel Management in 2015, and obtained the personal information of millions of government workers.

There are also questions of how long the government should hold on to the biometric data it collects when people enter and leave the country. Right now, Homeland Security discards biometric data of U.S. citizens after verifying it against that stored on their passports but, in the case of foreigners, stores the data for 75 years.