No surprise.
Via New York Times:
WÜRZBURG, Germany — The Islamic State claimed responsibility on Tuesday after a 17-year-old Afghan who came to Germany as a migrant attacked passengers on a regional train with an ax before he was killed by the police, a development that is likely to intensify fears that the huge influx of migrants poses a security threat.
The announcement, in a bulletin issued in Arabic and English via its Amaq News Agency, came after German authorities said that investigators had found a hand-drawn flag of the Islamic State, also known as ISIS or ISIL, in the room of the Afghan teenager, along with notes in Pashto indicating that he might have been self-radicalized.