The Department of Homeland Security says it expects to use facial recognition technology on 97 percent of departing passengers within the next four years. The system, which involves photographing passengers before they board their flight, first started rolling out in 2017, and was operational in 15 US airports as of the end of 2018.

The facial recognition system works by photographing passengers at their departure gate. It then cross-references this photograph against a library populated with facesimages from visa and passport applications, as well as those taken by border agents when foreigners enter the country.

Facial recognition has identified 7,000 passengers who’ve overstayed their visas

The aim of the system is to offer “Biometric Exit,” which gives authorities as good an idea of who’s leaving the country as who’s entering it, and allows them to identify people who have overstayed their visas. Quartz notes that US authorities have traditionally relied on airline flight manifests to track who’s leaving the country.

Since the introduction of the current system, facial recognition identified 7,000 passengers who overstayed their visas on the 15,000 flights tracked. The US Customers and Border Protection (CBP) estimates that over 600,000 people overstay their visas every year, an offense that carries a maximum penalty of a 10-year ban from entering the US.

Critics argue that building up a database of millions of people’s photographs is a threat to civil liberties. Once you have the database, it would be easy to share it with other agencies, effectively turning it into a search tool for all law enforcement.

The current iteration of the system first entered trials in 2017 on a single flight between Atlanta and Tokyo. It was originally planned to roll out more widely at the beginning of 2018, but its implementation was fast-tracked by the Trump administration and was expanded to more airports in the summer of 2017.