While it is clear, to anyone willing to take a long hard look at the evidence, that the U.S. government and military, with Cheney and Rumsfeld at the higher echelons, were involved in planning and staging the attacks...
http://www.911lies.org/home.html
...the hi-jackers were mostly alleged to be from Saudi Arabia.
Many of the named hijackers were then reported by the BBC to be still living in Saudi Arabia.
911 hijackers still alive? Fake Osama tape.
http://www.youtube.com/watch?v=ylNgK8k2kt8
Also, in the months leading up to 911, National Security Advisor Condoleeza Rice received numerous warnings of Al Quaeda plans to attack, from numerous foreign intelligence sources such as Russia, France, Egypt and others, including the specific nature of the attacks (putting the lie to statements by Bush and Rice that, "there was nobody in the administration that could imagine planes being flown into buildings.")
So there is evidence of government complicity, AND evidence of a foreign plot - no one may ever know what really happened and how, and who all was involved - one hypothesis is that the known real plans for hijackings were merged with the actual implementation of the attacks by the U.S. government and military, including the controlled demolitions of the WTC towers.
911 Myth and Reality - David Ray Griffin
written transcript http://blog.lege.net/content/911-Myth-Reality-Griffin30mar06.html
Includes a good discussion of reasons and justifications why many people simply will not look at 911 evidence pointing to government complicity.
video http://video.google.com/videoplay?docid=5382746000790234530&q=david+ray+griffin+myth+reality&hl=en