I believe it was a conspiracy.
It is so difficult in a country like the US to let an airplane fly so low in a place with so many skyscrappers around.
Everything seems so wrong in this story, in a country with so many security levels...
Now it seems like something that would never happen.
When it happened though, there had been no real safety features on a plane. The doors to the cockpit weren't locked, barely any screening at airports, no air marshals, no large scale intelligence at airports to be looking for this sort of behavior.
If we look at it from a lens of today, one that is pretty much relying on all of the intelligence we have today and the security features it's going to be hard for you to see it happening.
But it did happen, its' not a conspiracy theory. Two planes hit the WTC. One hit the pentagon, and another was going for the White House and it was taken over by the people on board.
What else is there to know about it?
No real safety features? Bull. There are all kinds of safety features everywhere on aircraft and in their design. What you are not considering is the viewpoint of an enemy, who searches for a weak point and then exploits it. At any moment, any year, such a person exists and is your enemy and is searching for weak points.