There are plenty of unanswered questions surrounding this event.
How did the steel melt at such a low temperature?
How did a building designed to withstand a hit from a much larger plane come down at free fall speeds?
Why did firefighters hear explosions below them when they were entering the building to save lives?
How did building #3 fall?
Why was the only piece left from the plane was a paper passport that fell directly on the ground by the building?
Why was NORAD told these planes veering off course were for a training exercise? Their fighter jets are ready to accost any plane that come off flight plan.

Any many more