The government? The one that we "elected" (election between two preselected candidates/puppets? Really?). Or maybe the real one, who's behind the scene, the puppet-masters?

If this was an inside job, there are not so many people (especially public people) who were behind it. They just needed to hire some Arabs (through several proxies) and train them to intercept the airplanes. And I'm sure those Arabs didn't know who was really behind it (they were sure it was Osama who could be an agent or a legend). Then make sure the secret services somehow fail to do their job at time X. Then eliminate/threat/silence those people at lower levels who couldn't understand some actions/lack of actions and were suspicious about that.

It isn't so hard for people with sufficient power and money, don't you think?