My belief about Sept 11, is that Al Qaeda terrorists who on other occasions have targeted the US, committed these acts. The notion that the US government and other governments around the world knew that something like this could happen is absolutely true. Please read about the
Bojinka Plot that was circa 1995. Quite frankly good ol US arrogance got in the way, just the thought that something of this scale could happen without being stopped first was prob unfathomable to most in CIA, FBI and government circles.