Humzah Mashkoor had just cleared security at Denver International Airport when the FBI showed up. The agents had come to arrest the 18-year-old, who is diagnosed with a developmental disability, and charge him with terror-related crimes. At the time of the arrest, a relative later said in court, Mashkoor was reading “Diary of a Wimpy Kid,” a book written for elementary school children. Mashkoor had gone to the airport on December 18 to fly to Dubai, and from there to either Syria or Afghanistan, as part of his alleged plot to join the Islamic State. The trip had been spurred...