But if you have limited time, here are the main takeaways.
Potential leads went unpursued for years.
An investigator found evidence that suggested Omar al-Bayoumi, a mysteriously well-connected Saudi student who knew two of the hijackers might have had prior knowledge of the attacks, even though senior U.S. officials had essentially exonerated him.
In a trove of seemingly disorganized evidence taken from Bayoumi’s home in Birmingham, England, in 2001, [a] detective found a spiral notebook that contained a hand-drawn aviation diagram of a plane descending to strike a spot on the ground. An F.B.I. agent who had studied aeronautical engineering concluded that the diagram showed a formula for an aerial descent like the one performed by Flight 77, the jet that Hazmi and Mihdhar hijacked, before it struck the Pentagon. Apparently, the notebook and its contents went unnoticed after Bayoumi’s detention and hadn’t been looked at again.
A former supervisor of the investigation said he thought the finding would have been more significant if it had been discovered in the fall of 2001.
“That would have been harder evidence,” the supervisor, Joseph Foelsch, said. “If not a smoking gun, a warm gun.”
Telephone records that were reanalyzed years later revealed multiple calls among Mr. Bayoumi; Fahad al-Thumairy, a Saudi diplomat and imam; and Anwar al-Awlaki, a Yemeni-American imam who was killed in a drone attack in 2011.
The C.I.A. interfered with a planned F.B.I. investigation, officials say.
In 2010, the F.B.I. planned to place two Saudi religious officials from the kingdom’s Ministry of Islamic Affairs under full-time surveillance. The bureau had previously found that their travel overlapped with movements by the hijackers and people believed to be supporting them, and that they had ties to suspected militants. The two men sought new visas to study English in the United States, which officials feared could be cover for something nefarious.
But, F. B.I. agents believe, the C.I.A. intervened before the surveillance could happen.
The episode, which has not been previously reported, ended abruptly. In the Saudi capital of Riyadh, C.I.A. officers objected strongly to the F.B.I. plan, one former official said. “They didn’t want to give the Saudis a black eye by letting these guys walk into a trap,” the former official said. For reasons that remain unclear, the two Saudis canceled the visit at the last minute. F.B.I. officials suspected that someone in the Saudi government had been warned.
A fabricated child.
To gain the trust of a reluctant source, investigators lied and told a Saudi man he had a previously unknown child. The investigator employed a graphics editor to fabricate an image of a 5-year-old using images of the man and his wife.