Masses, including me, now believe that the so-called Huge Terrorist attack in New York on September 11, 2001 was either a faked massacre in the United States that was devised as a portal for invasion into two Muslim countries or some other truth is yet to be unveiled. I could never make out the reason for highly unexpected failure of all 16 US Intelligence agencies, Air Force, National Security Council, NORAD, and security agencies of NATO Allies; the subsequent reports on TV, internet about delayed investigations and the unacceptable results publicized thereof, infiltrated with gaps and flaws, made my belief even stronger. I had always believed US security plan was indestructible and so I believe now that this happened because US wanted it so.
On the other hand, the US officials declared Al Qaeda as responsible and issued the public Commission report on July 22, 2004, trying to give reasons for the happening of this tragedy (9-11commission.gov) by hands of Al Qaeda; this document has been subjected to many controversies, most of which are unanswered yet.
The Official report claimed that Air Craft Navigators failed to detect the hijacked planes either because of the Military exercise going on that day or due to transponders that turned off somehow. They also admitted that the planners of this attack took advantage of lack of communication between US military and Air traffic control (2011, bbc.co.uk).