A "possible terrorist threat" shut down Rock am Ring, one of Germany's biggest music festivals, Friday near Nuerburg, AFP reports. Police say they have "concrete elements, in the light of which a possible terrorist threat cannot be ruled out." But no other details were released. According to the Guardian, thousands of people were asked to leave the festival calmly "in order to help police investigations." It's unclear how many people were in attendance Friday, but about 90,000 were expected over the three-day festival. Organizers hoped the festival would resume Saturday. Rock am Ring had already increased security, adding 1,200 additional staff, after the recent Manchester bombing at an Ariana Grande concert. (Rock am Ring was also suspended last year after dozens were injured by lightning.)