Part I is here.
In Cloak and Gown his book on the CIA, Robin Winks writes this about Sherman Kent's* view of the realities of of intelligence work.
[There are] no secret documents in the romantic sense of the words. On any important subject, there is no single document or even group of documents that contain "the secret." No spy could know enough to spot such a document if it existed, and no vacuum cleaner approach to espionage, even should it gather up two or three documents of the highest importance, would lead without all the analytical skills of the humanists to any valid conclusions. Documents do not speak: they do not declare that they are "the offbeat thoughts and recommendations of a highly-placed but erratic advisor," not a draft intended only for discussion, not a record of a decision rescinded orally the next day.
Unfortunately, many journalists still write as though there was some document the FBI could have obtained that would have revealed the whole conspiracy on 8-1-01.
I know of no evidence that Atta and his handlers had any sort of detailed written plan that they distributed to the other 18 hijackers in the summer of 2001. In fact, bin Laden indicated that many of the hijackers did not know they were going to crash the planes. So unraveling the whole plot would have required arresting Atta or one of the pilots and getting them to confess. Yet, what laws had Atta broken on 8-15-01 that would have justified that arrest?
Even if conspiracy to hijack could have been proven against most of the 19, would there have been a clear picture that they intended to use planes as giant bombs? Or would the reasonable conclusion have been that they were trying to force the release some prisoners as terrorists had often done before?
If the latter interpretation had prevailed, isn't it likely that al Qaeda would have kept trying for a 9-11 style attack? After all, they kept trying after the break-up of the New York terror cells and the arrest of Ramzi Yusef. The problem-- the planners-- were beyond the reach of the FBI and CIA. Stopping one conspiracy in New York offered no protection from the next. (Atta, for example, had no role in any of Ramzi Yusef's earlier plots; he was, recruited after the New York cells and the Manila plots were rolled-up.)
* Kent was the author of the book Strategic Intelligence for American World Policy and the towering figure in the creation of the CIA's analysis and estimate functions.
UPDATE: Donald Sensing comments here.