AHMED HIKMAT SHAKIR IS A shadowy figure who provided logistical assistance to one, maybe two, of the 9/11 hijackers. Years before, he had received a phone call from the Jersey City, New Jersey, safehouse of the plotters who would soon, in February 1993, park a truck bomb in the basement of the World Trade Center. The safehouse was the apartment of Musab Yasin, brother of Abdul Rahman Yasin, who scorched his own leg while mixing the chemicals for the 1993 bomb.When Shakir was arrested shortly after the 9/11 attacks, his "pocket litter," in the parlance of the investigators, included contact...