There is really no doubt at all.  

The "conspiracy" does not have to be that large.  A single commando team, doing the work ahead of time.  Plus a few--just a few--high-level officials to give them access.  

Of course such a team would have to be crazy--virtual psychopaths . . . Not a counter argument:  It merely moves Bush and Cheney up on the list of suspects.  

The Fates are kind.