11 years later...

[hack89]

21. Here are the accounts

Omar Campo, a Salvadorean, was cutting the grass on the other side of the road when the plane flew over his head. 'It was a passenger plane. I think an American Airways plane,' Mr Campo said. 'I was cutting the grass and it came in screaming over my head. I felt the impact. The whole ground shook and the whole area was full of fire. I could never imagine I would see anything like that here.'"
- "Pentagon Eyewitness Accounts." The Guardian, 12 Sep 2001

"'I saw the tail of a large airliner. ... It plowed right into the Pentagon," said an Associated Press Radio reporter who witnessed the crash. 'There is billowing black smoke.'"
- "America's Morning of Terror." ChannelOne.com, 2001

"'I saw the tail of a large airliner. ... It plowed right into the Pentagon," said an Associated Press Radio reporter who witnessed the crash. 'There is billowing black smoke.'"
- "America's Morning of Terror." ChannelOne.com, 2001

"Frank Probst, an information management specialist for the Pentagon Renovation Program, left his office trailer near the Pentagon's south parking lot at 9:36 a.m. Sept. 11. Walking north beside Route 27, he suddenly saw a commercial airliner crest the hilltop Navy Annex. American Airlines Flight 77 reached him so fast and flew so low that Probst dropped to the ground, fearing he'd lose his head to its right engine."
- "A Defiant Recovery." The Retired Officer Magazine, January 2002

Many more at link

http://urbanlegends.about.com/library/blflight77w.htm

Now lets apply some logic here. The Pentagon is in a large urban area next to freeways that were packed for the morning commute. The plane flew over one of those freeways. The plane was seen by many people.