http://www.9-11commission.gov/report/911Report_Ch1.htm
Reagan National controllers then vectored an unarmed National Guard C130H cargo aircraft, which had just taken off en route to Minnesota, to identify and follow the suspicious aircraft. The C-130H pilot spotted it, identified it as a Boeing 757, attempted to follow its path, and at 9:38, seconds after impact, reported to the control tower: "looks like that aircraft crashed into the Pentagon sir."147
The aircraft that spotted the "black smoke" was the same unarmed Air National Guard cargo plane that had seen American 77 crash into the Pentagon 27 minutes earlier. It had resumed its flight to Minnesota and saw the smoke from the crash of United 93, less than two minutes after the plane went down.