Scott P. Cook  was at work on the fifth floor of the Portals building, at 1280 Maryland Avenue SW, Washington DC. immediately opposite to the Pentagon across the River.
  -  "As we watched the black plume gather strength, less than a minute after the explosion, we saw an odd sight that no one else has yet commented on. Directly in back of the plume, which would place it almost due west from our office, a four-engine propeller plane, which Ray later said resembled a C-130, started a steep decent towards the Pentagon. It was coming from an odd direction (planes don’t go east-west in the area), and it was descending at a much steeper angle than most aircraft. Trailing a thin, diffuse black trail from its engines, the plane reached the Pentagon at a low altitude and made a sharp left turn, passing just north of the plume, and headed straight for the White House.
  All the while, I was sort of talking at it: "Who the hell are you? Where are you going? You’re not headed for downtown!" Ray and Verle watched it with me, and I was convinced it was another attack. But right over the tidal basin, at an altitude of less than 1000 feet, it made another sharp left turn to the north and climbed rapidly. Soon it was gone, leaving only the thin black trail.
http://www.clothmonkey.com/91101.htm

  Keith Wheelhouse, of Virginia Beach, spotted the planes first. The second plane looked similar to a C 130 transport plane, he said.   He believes it flew directly above the American Airlines jet, as if to prevent two planes from appearing on radar while at the same time guiding the jet toward the Pentagon.  insists he was not confused by other air traffic.
    A Norfolk-based FBI agent interviewed him, Wednesday evening.
    (abstracted from the ARLINGTON Daily Press; Newport News; Sep 14, 2001)

In a TLC documentary the pilot of the C-130 was identified
NARRATOR:
By chance a C-130 is already in the skies above Washington. The
plane everyone is looking for is right in front of them.
National Guard Lt. Col.Steve O'Brien.
We were at about 3,500 feet at the time that I first noticed this commercial airliner in our 12 o'clock position in about a 45-degree bank, which is unusual for a large aircraft to be descending and turning at a 45-degree bank turn like that, so that really got our attention. The next thing we saw was a fireball and a big plume of smoke.
From a mail message from 'Isopodia'.