Some kind of Photoshopped fuzz blob thing - done to hide the missile debris being shot from the building. You can see the obvious messing around of the pixels.
From: http ://thewebfairy(dot)com/911/missileout/index(dot)htm
View the real thing here:
http://www.dailymotion.com/chipmuck/video/x3quyw_plane-in-missile-out_tech
