Here is an outlandish thought: what if Saddam Hussein was really behind 9/11 and the Bush administration has proof of this, but cannot disclose the evidence that points to this without compromising themselves (and leaving themselves open to charges of treason, for example)? The irony is that even if they found some way to disclose this without allowing any blame to fall on themselves, it is safe to say that nobody would believe them (their credibility being what it is).