Utter horseshit. There was lots of plane wreckage. It just happened to be contained within the confines of that massive section of the pentagon itself. Lots of it would burn up, melt, get mixed up with upteen thousand tons of reinforced concrete... Not to mention that the whole area is classified as hell to begin with. This might all be stemming from the modified footage of the wreck wherein the feds clipped out the frames showing the plane coming into the building, leaving it all to look like the building just blew apart spontaneously.