One Saturday morning in 2015, Sabine Lappe's telephone rang. It was her son Christian calling to say that he and his young wife were on their way to Syria to join terrorists fighting for the so-called Islamic State. He never returned. He was killed in 2017. DW's Matthias von Hein met with his mother, who lives in the German town of Dortmond, to talk about her son and his path to radicalization.