> It was a SNAFU of competing interests and stupid bureaucracy.
I hear where you're coming from, but after 20 years, there are several parts of the story for which I can no longer believe this was the case, or use this explanation to give the benefit of the doubt. (There are many other parts for which it does make sense.) I'll save myself looking like an internet crank and just suggest you give the podcast I linked to in [2] above an earnest listen. If you do, you may, like me, wonder from how high up the order came to personally stop the FBI man in question, Mark Rossini, from telling his superiors at the Bureau. It's at least George Tenet's former assistant, sent down to run Alec in '99.
I'm not sure what "all this information" refers to - no one knows exactly how much the institutions in question knew beforehand - but it's there in Chapter 6 of the 9/11 Commission Report (albeit buried in a footnote) that 40-50 people at Langley knew enough to stop the resulting plot had they wanted to, that senior CIA employees prevented the FBI from knowing about 2 Al Qaeda terrorists coming to and living in the US, and that specific upper management (whose names are now publicly known) told lower-level personnel that the FBI had been told when it's known that they hadn't. This is corroborated by serious, non-crank public servants who were there in the room when this stuff went down.
The result, for me, is that my default position on issues of war, terror, and other bloodshed has moved from "organizational dysfunction is the norm" to "I may never truly know, but these people are likely acting maliciously (to some degree) and/or lying about it (to some degree)". It's like, these people have jobs, and they do stuff. They're serious people, like many of us on HN, and they're competent a lot of the time. To the extent that we even know their track record, they have a track record: Iran, Chile, Guatemala, Congo, Iran again (Stuxnet), etc., etc.
So it's reversed the Sagan standard for me, in other words: if "extraordinary claims require extraordinary evidence", the idea that the professionals to whom we give vast sums to be excellent at lying, cheating, stealing, killing, and betrayal - spies, in other words - should be given a sort of "Mister Magoo" benefit of the doubt when lots of people die -- that's, for me these days, the more extraordinary claim.