The report of the commission investigating the attacks of September should be required reading for every adult American. One familiar name is Richard Clarke (the former counter-terrorism guy the Bushies tried to smear after he pointed out that attacking Iraq made us more vulnerable to terrorism).
It's unambiguously clear that Clarke (and to a lesser degree George Tenet, director of the CIA) absolutely understood the danger posed by Osama Bin Ladin as early as 1998. In Clinton's White House, he was effectively a member of the Cabinet, and he was completely obsessed with killing Bin Ladin (Chapter 4 of the Commission's Report). He was ridiculously focused, probably to the point of annoying people. But he was right all along. He sent a memo to Condoleeza Rice: "[A]re we serious about dealing with the al Qida threat? . . . Is al Qida a big deal? . . . Decision makers should imagine themselves on a future day when the CSG [Clarke's group in the White House] has not succeeded in stopping al Qida attacks and hundreds of Americans lay dead in several countries, includig the US. What would those decision makers wish that they had done earlier? That future day could happen at any time." This memo was dated September 4, 2001..... (p. 212 of the Report)
Blame goes to Louis Freeh (and to the extent she was his boss Janet Reno), who did a poor job at the FBI on fighting terrorists. FBI agents were confused about what they were allowed to share with each other and they ended up erring on the side of not sharing information.
Blame also goes to Rice, an expert on the Soviet Union, who demoted Clarke as soon as the Bush adminstration took over (p. 200) and stopped listening to him.
More blame should go to John Ashcroft, who speechified about how important it was to fight terrorists, but didn't make them a real priority (pp. 209, 265). [Of course, Ashcroft did make it a priority to make sure he was never seen in a picture with the statue of a woman's breast. Whew! Good job!]
Essentially, the Bush administration just didn't make Osama Bin Ladin a high priority--arguably he was a higher priority for the Clinton administration than for the Bushies. Oops.