German authorities say they have arrested a man who returned in August from a long stay in Syria and says he spent time there with the extremist Islamic State group.

Police and prosecutors said Friday the 21-year-old from Muelheim an der Ruhr in western Germany, who wasn't identified in line with German privacy rules, was arrested on suspicion of preparing acts of violence.

Authorities say the man was suspended from a vocational college at the beginning of this year after showing up with a prayer rug and attempting to "proselytize" among fellow students.

He was put under observation after he returned from Syria. Prosecutors say officials searched his apartment and arrested him on Wednesday after noticing "peculiar behavior." They didn't elaborate.