It was an extraordinary coup: in the late 1990's, federal prosecutors sat down to interview their first major informer against Al Qaeda — Jamal Ahmed al-Fadl, a former payroll manager for Osama bin Laden who would become the government's chief witness in its first trial of Qaeda operatives. But it was followed by an extraordinary blunder: for two years, even as the authorities cloaked Mr. Fadl in the secrecy of the federal witness-protection program, keeping him in undisclosed locations and communicating with him in videoconferences through a special telephone hookup, many hours of those conversations were recorded on videotape. The...