http://www.freedomunderground.org/memoryhole/pentagon121.swf it started as a french conspiracy and leaves out a huge piece of
information. there were pieces of the plane there. the first
photojournalist on the scene took pictures of them. they interviewed a bunch of eyewitnesses and the wife of the pilot. apparently, the wings of the plane collapsed and just the fuselage went through the building. but the fire, since the plane only flew a short distance and was full of fuel, was incredibly intense. it burned its way through the building.
it's an interesting theory nonetheless.