In summer 2003, Shahawar Matin Siraj, then 21, met Osama Eldawoody, a nuclear engineer twice his age. To Mr. Siraj’s delight they struck up an unlikely friendship — never before had someone this sophisticated taken him so seriously. At the older man’s encouragement, Mr. Siraj became entangled in a plot to place a bomb in Herald Square subway station. He would later want out of the plan, but it was too late: Mr. Eldawoody, it turned out, was one of thousands of informants recruited by the police and the F.B.I. after the Sept. 11 attacks. Today on The Sunday Read, did the U.S. government’s network of informants create plots where none existed?