This from The Independent today:
http://news.independent.co.uk/world/americas/story.jsp?story=515270
"The Bush administration has been put on the back foot by allegations that senior officials - perhaps even Mr Bush himself - were provided with considerable information warning of an imminent attack by al-Qa'ida and that they failed to act. Mrs Edmonds said yesterday: "What are they are afraid of? If I am not allowed to give evidence, the families will not get the information I have; that will be that."
She said it was wrong for the Bush administration to claim it wanted a full investigation. "If there is transparency, there is going to be accountability and that is what they don't want." "
This shows
The Bush administration knew of the coming attacks.
They knew aircraft were going to be used.
They did nothing to prevent this, and may have hindered investigation procedures.
Why?
Possibly because, they wanted the attacks to take place.
Why? Because then they would have minimal resistance to their plan to attack Iraq.