In a surprisingly low-key interview, Richard Clarke, former White House Counterterror "Tsar" to Presidents Clinton and Bush, goes on record about what he believes happened at the CIA in the run-up to the 9/11 attacks, accusing then-CIA Director George Tenet and two of his deputies of deliberately not informing the White House, FBI, and Defense Department about two future hijackers inside the US, then covering it up after the 9/11 investigations.
Part 2 is "Who is Rich Blee?" an investigative podcast documentary, written and produced by Ray Nowosielski and John Duffy, fleshing out more of the details in this interview. This podcast is key to understanding the role of the CIA/FBI Alec Station group in the 9/11 attacks and the official cover-up.
There was supposed to be a follow up to this investigative report, but after the CIA issued legal threats to the two journalists working on the series, the report sadly appears to have been successfully derailed.
An excellently written article on this subject is:
9/11 and the Question of “Failures” - Deliberate or Incompetence?
Also see: 9/11 History Timeline: A 25 Page Summary from Verified Mainstream News Reports