Officially, the FBI agents who swarmed Donald Ray Morgan at Kennedy Airport this month were there to arrest him on a mundane gun charge. But they whisked him away to their Manhattan office and grilled him for two hours on an entirely different topic: Islamic State extremists. Over and over, they asked Morgan, a 44-year-old North Carolina man, converted Muslim and author of pro-extremist tweets, whether he had traveled to Syria to support the militant group. More important, they wanted know whether he could identify any fighters with U.S. ties who had left the region to return to America. The...