So, perhaps they can stay in Germany where they will feign remorse, fade off the radar and then....strike?
Until somebody does something, not much you can do to them.
The history of surveillance over guys like that is not good. Note the Paris terrorists, all of whom raised their hands some time ago...and then were ignored.
The threat of being stripped of a German passport is enough. After that, they’re nobody. Maybe they can get one from ISIS. Sort of an instant “go to jail now” card.
The Bundesnachrictendienst (BND) is far better at surveillance than is given credit for. The USA is quite dependent on them right now due to the continued gutting of the CIA . . . not that Obama listens when he doesn’t feel like it, mind you. The Bundesamt für Verfassungsschutz (BfV) is no joke either.