And as the cross-border conflict in Syria and Iraq continues to rage, it is luring hundreds of would-be jihadis from Europe, including radicalized converts like the ex-rapper. Although the German authorities have said they see no indications an attack is imminent, they are attempting to track fighters returning from the war zone.

Currently, there are an estimated 400 Muslim extremists from Germany in Syria, either directly engaging in fighting or otherwise supporting the Islamists. Altmüller said roughly 10 percent of them were German converts to Islam. According to intelligence reports, some 40 have been killed in the conflict.

That was almost the fate of the 38-year-old Cuspert after he was seriously injured in an airstrike by Syrian forces in September 2013. But he survived, and being wounded apparently helped him win respect among the extremists. He eventually swore allegiance in an online video this spring to Abu Bakr al-Baghdadi, who has since declared himself caliph of the repressive Islamic State.