Police have used facial recognition technology to arrest a man for the first time in the UK.

The arrest was made May 31 but police didn't release many details, probably because the investigation is ongoing.

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South Wales Police have been trialling the technology, known as AFR (Automatic Facial Recognition), for the past 18 months. And UK police have been testing facial recognition technology for a while, raising privacy and security concerns among civil rights organisations and members of the public.

At the end of May, law enforcement announced a partnership with the company NEC to test AFR during the Champions League finals week in Cardiff. Trained officers monitored "the movement of people at strategic locations in and around the city centre".

A number of camera positions were set up to identify people who are on the police's watch list, either because they're suspects, missing persons, or persons of interest.

The arrest was "of a local man and unconnected to the Champions League," a South Wales Police spokesperson told Mashable.

The man's face was probably included in the police's Niche Record Management system which stores 500,000 custody images.

Police said the UCL final "has clearly provided a perfect testing ground" for the AFR trial with NEC, which has been working on real-time facial recognition along with UK police.

"The world we live in is changing and with that comes a need to change the way we police," Assistant Chief Constable Richard Lewis said.

According to Alun Michael, South Wales police & crime commissioner, AFR helps law enforcement in early intervention and prevention of a crime "by allowing us to identify vulnerability, challenge perpetrators and reduce instances of offending within environments where the technology is deployed."

While this is the first arrest attributed to facial recognition technology, it's been in use in the UK for a while now.

In 2015, Leicestershire police scanned the faces of 90,000 festival-goers at Download Festival, checking them against a list of wanted criminals across the country. It was the first time anywhere in the UK that facial recognition technology — NeoFace — was used at a public outdoor event.

Privacy campaigners — and Muse frontman Matt Bellamy — expressed their fury at authorities after they casually mentioned the use of the surveillance project on Police Oracle, a police news and information website. Police didn't use any other method to warn festival-goers about the controversial initiative.

Facial recognition is also widely used in the U.S. enforcement agencies.

According to a study published by researchers at Georgetown University's Center on Privacy and Technology, about half of American adults — 117 million Americans — are already in a "law enforcement face recognition network."