Why did UK police scan the faces of 100,000 concert goers?

About 100,000 attendees to Download Festival 2015 in the U.K. had their faces scanned by police-operated recognition systems. The scanned images were compared with a database of images taken from across Europe, though the purpose of the scanning is unknown. Some speculated the information would be used to catch smartphone thieves, while others surmise that police are after more serious criminals. In any event, the use of such new and powerful technology illuminates changing paradigms in the realms of both privacy and crime-fighting.