The US Department of Homeland Security expects to use facial recognition technology to scan 97% of passengers at airports in the next four years. The method involves photographing of passengers before they board the flight and cross-referencing it against a library of images from visa and passport applications, and those taken by border agents, The Verge reports. The technology is used to keep track of those who have overstayed their visas. However, it is being criticised for hampering citizens' privacy. The system was already operational at 15 US airports by the end of 2018.