Aus 9/11 Wiki
John Bowman, a retired Marine lieutenant colonel and a contractor, was in his office in Corridor Two near the main entrance to the south parking lot. "Everything was calm," Bowman said. "Most people knew it was a bomb. Everyone evacuated smartly. We have a good sprinkling of military people who have been shot at."
He was inside the building and did not see the flight of the aircraft. He actually thought it was a "bomb".
Source List: Bart