The prime suspect in the alleged German terror plot to blow up hundreds of people may have had contact with Mohammed Atta, the suicide pilot who smashed a hijacked plane into the Twin Towers. German police reports are vague about the encounter, but it has helped investigators to trace Fritz Gelowicz’s path from average Bavarian schoolboy to Islamic radical. His case has also highlighted the curious and sinister role of Neu-Ulm — a small township in the Roman Catholic heart of southern Germany — as a cradle of Islamic extremism. The connection between Gelowicz, 28, who was arrested this week,...