I dont know of any evidence that Bush et al were behind 9/11.
I do believe they demonstrated malfeasance in office..being so driven by an ideological pursuit of a different foreign policy priority.... expansion of Reagan's missile defense program.
They were presented with a memo in Jan 01, days after taking office, with an assessment of the seriousness of the al Queda threat and very broad suggestions for a course of action.
"We urgently need . . . a Principals level review on the al Qida network."
Read the full memo (pdf)
more: The National Security Archive
That memo sat idle for nine months, while the Bush administration single-mindedly focused on their top foreign policy initiative of missile defense.