In the summer of 2002, congressional investigators probing the September 11 terror attacks made a startling discovery. A college professor and longtime FBI informant in San Diego had dealt extensively with two of the 9/11 hijackers. The informant became close to the future terrorists after he'd rented them rooms in his house. The connection raised plenty of questions: What did the informant know about the activities of his housemates? And why hadn't the FBI said anything about the connection?

The discovery led to a closed-door confrontation between the FBI and Florida Sen. Bob Graham, co-chair of the joint House-Senate panel investigating 9/11. Convinced that the bureau was stonewalling, Graham tried to slap the FBI's chief counsel with a subpoena to produce the informant. "With the subpoena still in hand, I approached him, holding it inches from his chest," Graham writes in his new book, "Intelligence Matters," which deals with his efforts to get to the bottom of the 9/11 attacks. "He leaned back from the subpoena as it if were radioactive." The FBI counsel asked for extra time to see if something could be worked out. In the end, the FBI refused to allow Graham and his colleagues to question a crucial witness.