Alec Station scandal: CIA leadership refused to tell FBI the hijackers had entered the country
The plausible reason why Alec Station (the CIA's Bin Laden tracking effort before 9/11) didn't warn the FBI is that if they had done so, the attacks could not have happened. It's similar to the experience that rank and file FBI agents had when they tried to get a warrant to search Mr. Moussaoui's computer after he was arrested in August 2001 (when the flight school he was attending called the FBI, worried - correctly - about his intentions). The probable cause for the warrant was removed by headquarters and the search didn't happen until after the attack. If they had been allowed to search his computer beforehand, the attacks could not have happened.
"The power to see to it that regular government operations don't occur is one of the greatest controls over power you can wield in a government."
-- Colonel Fletcher Prouty, Pentagon liaison to CIA
Kevin Fenton, "Disconnecting the Dots: How CIA and FBI officials helped enable 9/11 and evaded government investigations"
is a book mostly about Alec Station. In the unlikely event there is a real investigation Disconnecting the Dots would be a good place to start. Fenton participated in the History Commons website documenting the 9/11 timeline, formerly called the Center for Cooperative Research. www.historycommons.org
Newsweek runs one of the best stories about how 9/11 was enabled
www.newsweek.com/2015/01/23/information-could-have-stopped-911-299148.html
It's about the "Alec Station" scandal, the CIA Bin Laden station which refused to tell their ostensible partners (FBI) that the hijackers had entered the country. Suppressed warnings aren't a popular topic for the "alternative" media nor the 9/11 Half Truth Movement.