Judge Pauley cites the 9/11 Commission Report for his statement that telephone metadata “might have permitted the N.S.A. to notify the [F.B.I.] of the fact that al-Mihdhar was calling the Yemeni safe house from inside the United States.” What the report actually says is that the C.I.A. and the N.S.A. already knew that Al Qaeda was in America, based on the N.S.A.’s monitoring of the Hada phone. If they had told the F.B.I., the agents would have established a link to the embassy-bombings case, which “would have made them very interested in learning more about Mihdhar.” Instead, “the agents who found the source were being kept from obtaining the fruits of their work.”

The N.S.A. failed to understand the significance of the calls between the U.S. and Yemen. The C.I.A. had access to the intelligence, and knew that Al Qaeda was in the U.S. almost two years before 9/11. An investigation by the C.I.A.’s inspector general found that up to sixty people in the agency knew that Al Qaeda operatives were in America. The inspector general said that those who refused to coöperate with the F.B.I. should be held accountable. Instead, they were promoted.

Yes, the F.B.I. could have stopped 9/11. It had a warrant to establish surveillance of everyone connected to Al Qaeda in America. It could follow them, tap their phones, clone their computers, read their e-mails, and subpoena their medical, bank, and credit-card records. It had the right to demand records from telephone companies of any calls they had made. There was no need for a metadata-collection program. What was needed was coöperation with other federal agencies, but for reasons both petty and obscure those agencies chose to hide vital clues from the investigators most likely to avert the attacks.