JOHN KING: Today, six years after 9/11, a mystery endures about just what happened in the skies over the White House that terrible day. A plane flew right over it, but why, and what was it? For conspiracy theorists, the image is a gold mine. It appeared overhead just before 10 a.m., a four- engine jet ... in the nation's most off-limits airspace. On the White House grounds and the rooftop, a nervous scramble. And still today, no one will offer an official explanation of what we saw. Two government sources familiar with the incident tell CNN it was a military aircraft. They say the details are classified. This comparison of the CNN video and an official Air Force photo suggests the mystery plane is among the military's most sensitive aircraft, an Air Force E-4B. Note the flag on the tail, the stripe around the fuselage, and the telltale bubble just behind the 747 cockpit area. MAJ. GEN. DON SHEPPERD (RET.), U.S. AIR FORCE: There are many commercial versions of the 747 ... that look similar, but I don't think any of them that have the communications pod like the ... Air Force E-4 does behind the cockpit. KING: The E-4B is a state of the art flying command post, 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. Ask the Pentagon, and it insists this is not a military aircraft, and there is no mention of it in the official report of the 9/11 Commission. [In] sum: the lack of any official explanation feeds an ominous conspiracy. This is from an online discussion about the plane on the web site 911blogger.com. "I have always thought these planes were exactly that, mission control for the 9/11 attack on our country."