It's rediculous.
First, the attack on the Pentagon occured during or just after rush hour in the DC area. I lived in that area until 2000, and drove on 395 which is hte freeway that goes closes to that side of the Pentagon (which is why that side was reinforced first, it's close, very close actually, to a major freeway), and at that time of day, there are thousands of cars and people nearby. Did that many people just imagine a plane flying over them and crashing into the Pentagon? That highway is pretty busy all day, so even if it was after rush hour, there would still have been plenty of people to see a plane crash into the building.
Second, I was there a month after it happened and drove past on 395. I saw the damage and how bad it was.
If it was some sort of conspiracy, how did they convince so many people that they saw a plane crash into the building, and how did they manage to brainwash so many employees (both actual employees and contractors) that they saw a plane crash into it?