The perpetrator of the German trail attack was likely a member of ISIS; or at the very least, he was inspired by the terrorist organization. ISIS members have been successful at infiltrating many European countries in order to carry out terrorist attacks throughout the continent. The train attacker was likely another member of ISIS.
The perpetrator of the German train attack was probably a member of ISIS. It is not unreasonable to make this assumption, particularly since most, if not all, similar attacks have been carried out by terrorist organizations sponsored by this group. Perhaps it's crying wolf, but the evidence is in and it is reasonable to conclude this.
He has not only declared himself a member, but ISIS has claimed responsibility. These are two very clear signs that the perpetrator of the German train attack was a member. If these things were not true, there would be no reason to believe that he was a member just because of circumstances.
Police now know that the attacker was a 17-year-old migrant who arrived in Germany without his parents last year and sought asylum in the country. While he may well have been motivated to carry out the attack by ISIS propaganda, he appears to have been acting alone and may have self-radicalized following the recent death of a friend in Afghanistan.