Four days after 9/11, Robert Mueller was summoned to the presidential retreat at Camp David. It was little more than a week since he’d become director of the Federal Bureau of Investigation.

That Saturday morning, Mr. Mueller gave President George W. Bush the FBI’s initial report on the attacks. The Pentagon/Twin Towers Bombing Investigation, or Penttbom, would become the largest ever conducted by the FBI. It had already identified the 19 hijackers as well as their roles, nationalities, travel documents and histories. The focus had turned to establishing links between the hijackers and al Qaeda.