From accuracy.org, former FBI agent Colleen Rowley on the 20th anniversary of the 9/11 attacks:

“Already on the day of September 11, 2001, some of us insiders knew some agonizing truth, that the horrible terrorist attacks of that day could have been easily prevented if FBIHQ had just read, shared and acted on the memos addressed to officials there. But I didn’t know the half of it that day, that other agencies like the CIA actually knew much more as their agents had been monitoring the meetings and developments of key Al Qaeda figures for a couple of years prior but were keeping that intelligence secret even from other counter-terrorism agencies.

Most importantly the CIA hid the fact from the FBI until it was too late that they knew Al Qaeda terrorists (who would become 9-11 aircraft hijackers and suicide pilots) were already in the country. Along with all the prior info they kept secret, Director of Central Intelligence George Tenet had been briefed with a ‘Fundamentalist Learns to Fly’ PowerPoint weeks before the attacks. (He later could not explain why as head of all U.S. national security intelligence, he still took no action.)

Source: 9/11 Cover-up: Whistleblower Coleen Rowley – accuracy.org