Robert A. Leonard,
a resident of West Springfield who works near L'Enfant Plaza, was in the HOV lanes of I-395 on his morning commute to work. His car passed the crest of the hill, at the point where Washington comes fully into view and the Pentagon is on the left:
"I looked in the rearview mirror to check the traffic and saw only a plane, flying very low. I followed it in my left outside mirror. I braked, looked out my left window and saw a large commercial aircraft aiming for the Pentagon."
"The aircraft, so close to the ground, was banked skillfully to the right, leveled off perpendicular to the Pentagon's southwest side, then went full throttle directly toward the building. The plane vanished, absorbed by the building, and there was a slight pause. Then a huge fireball rose into the sky."
Washington Post, 9/20/01(Lexis Nexis)
(text mirror) http://www2.hawaii.edu/~julianr/lexisnexis/leonard1.txt
Phone call to Robert Leonard 06/19/10 (Mp3 download)
Good call, Jeff!
[The story link didn't work for me, tho.]