Let's go with the official story for a second and see where it takes us on this one:
One of the close contacts of the 9/11 Hamburg cell, also with contacts to Osama Bin Laden, is a leader at a Mosque in Hamburg named Darkanzanli. He reportedly received $250,000 from a known Saudi intelligence front company in the 90s, and some even claim he chose the 9/11 Hijackers. Guess what? He was allowed to continue living freely in Hamburg after 9/11. Guess what else? The CIA tried through German intelligence to turn him into an informant before 9/11.
He is now in the news because ABC News has reported that one cell of the recent European terror plots has been traced to this same Mosque in Hamburg where Darkazanli is a leader. As shown in the ABC video, this Darkazanli is now wrapped up yet again in a new terror plot, and yet remains in Hamburg living freely.
One possibility could be that he is a CIA informant and for this reason is left alone. In any case, his true role seems as opaque as his name to english ears sounds.