This is what I, and most sane people, think happened.
9/11 happened. The planes, all of it.
A few CIA's in Washington, far away from Bush, were thinking: "Dang, we should have acted on this lead, even though we get thousands of leads a day just like this...woops"
The President and his buddies think: "Dang this sucks, we might as well make the most out of this crappy situation. Let's use it as a platform to push our own agendas."
Americans are thinking: "Man this is horrible. What if this happened to me? Are we going to get nuked? My town has a refinery/military base/Capital building in it... something is definitely going to happen to me. Save me Bush! We will follow you!"
Bush: "Well, ok!" (Patriot Act, Wars with oil revenue and dramatic, Shakespearean revenge plots, etc...)
No cover ups, just fuuck ups. On all of our parts.