Further witnesses confirmed that the prison guards, disguised in so-called ski-masks would appear in groups of three or four. The attacks mostly happened at night. According to the details, the guards struck the prisoners with their fists and with nightsticks. Some of the victims suffered serious injuries and broken bones.
Of course, this is in a civilian prison in Germany. In fact, one of the undercovered angles to the Abu Ghaibr story is that many of the perpetrators seem to have been prison guards in civilian life, and I suspect -- as previous posts here on more than one occasion suggest -- far worse behavior is routinely tolerated there.