Consider the following:

Having been explicitly told by their predecessors that they were going to have to keep their eye out for Al Qaeda terrorism, the administration specifically told our intelligence services they could not investigate Al Qaeda, and set up new protocols that prevented the flow of information between those agencies.

As the chatter increased to the point that an attack was expected, they went on vacation.

In her testimony before the commission, National Security Advisor Condi Rice bemoaned the absence of a mechanism for coordinating and collating between the various security agencies. There is, in fact, such a mechanism: the National Security Advisor. We have those election thingies every four years in order to try to choose people who are competent to handle these responsibilities, and this administration, for whatever reasons, has proven not to fit the bill. There is no proof that someone else would have prevented 9/11, but there is more than enough evidence that someone else might have. It is certain that almost anyone else would have tried. This administration did not.