IMPEACH BUSH

Shortly before 10 am on the morning of September 11, 2001, amid rumors of a fourth hijacked plane headed for Washington, DC, a mystery aircraft appeared in restricted airspace over the White House. There has never been an official explanation for this incident, which has provided abundant fuel for 9/11 conspiracy theories.

CNN has now learned from two government sources that the mystery plane was a military aircraft and has determined that the blurry image on video appears to match photos of the Air Force's E-4B, a specially modified Boeing 747 with a communications pod behind the cockpit.

"The E-4B is a state of the art flying command post," CNN explained, "built and equipped for one reason -- to keep the government running no matter what, even in the event of a nuclear war, the reason it was nicknamed the 'doomsday plane' during the Cold War."

9/11 Commission co-chair Lee Hamilton told CNN he was aware of the incident and that it had simply never seemed important enough to make it into the commission's report. He called conspiracy theories involving government complicity in 9/11 "ludicrous."

The plane was previously identified as the E-4B a year ago by one researcher on a forum associated with the 9/11 conspiracy film, Loose Change.

CNN acknowledges that, despite its identification, the absence of the aircraft from official investigations, together with the Pentagon's denial that it was a military plane and the insistence by the Pentagon, Secret Service, and FAA that they have no explanation for the incident, may continue to raise suspicions.