Many European citizens left the continent to join the Islamic State group. As the war against IS winds down, hundreds of them are still imprisoned in Kurdish-controlled regions of northern Syria. United States President Donald Trump has demanded European countries take back their citizensbeing held by the Kurds. Many of the returnees would likely end up in prison in Europe. Some politicians and experts worry that in Germany, authorities would then be confronted with the challenge of preventing Islamic extremists from turning the country’s prisons into breeding grounds for radicalization. According to Michael Kiefer, an expert on Salafism and radicalization prevention who teaches at the University of Osnabrück, that is a very real danger.