I have more than once thought of Thomas Friedman's observation that the scale of the attacks on 9/11 were due to a lack of imagination. OUR imagination.

I think it's pretty clear that, as George Bush was quoted, he didn't have OBL as a top priority. From what I've heard and read, however, the FBI were hardly at the top of their game and did overlook some real danger signs.

In 2001 a television show called "The Lone Gunmen", a short lived spinoff from "The X-Files" had as an episode a government plot involving flying a computer controlled airliner full of people into one of the twin towers. It has a pilot's eye view of the approaching towers at night, and at the last minute our hero takes control and is just able to clear them.