It would take at least a hundred people several days to set charges in the towers.
Somebody would have seen them, especially in Tower 7, where security would have to be pretty tight.
Have you ever tried to get into an office building? There's generally somebody somweplace who wants to know who you are and what you're doing there, and check some I.D. I would think this would be especially true of the World Trade Center, since the buildings were bombed in 1993.
I loathed Bush and Cheney, and I would put very little past them. But I don't believe there was any 9/11 conspiracy.