Murat Cem lived among pushers, killers, islamists. He was one of the most successful police informers in German history. And he knew the ISIS assassin Anis Amri who killed 12 people in Berlin - but police did not listen to his warnings. We did a video about his story and a book that was published by Der Spiegel's journalists Roman Lehberger, Fidelius Schmid and Jörg Diehl.
