Exactly, Husnaa and BKGZ! There are many cover-ups in and around that attack(9/11); as there are, again, many conspiracy theories of reasonable bases and conclusions regarding the same attack, its actors & masterminders. Just chose which appeals more to your perception and conscience. I, for example, don't believe that it was carried out by Muslims. One simple question is: how exactly did they find that out?
Beside, am not with Osama on many grounds, religiously speaking; yet I blame US more than anyone/thing for what he and his likes (e.g Boko Haram) are doing. Osama was a creation of US. On that same line, I can recall, Cuban former president, Castro saying (that) he believed Osama was a CIA agent. He might be wrong but whatever, US played a significant role in the making of violent Osama. Remember Soviet-Afghan war (1979-1989)?
And, as said Abu-Fatima, some more Osamas are coming aboard. That can only be avoided when US changes its foreign policies, especially to Muslim countries. And that requires more than just delivering lecture at Al-Azhar University or saying you (Obama) grew up with Muslims, attended their school, etc. That was a mere 'nostalgia'. Yah. Actions, not just words.