[WinOne4TheGipper]

But, but... You can't tell what it is in the video frames!!!

http://upload.wikimedia.org/wikipedia/en/f/f9/Pentagon_impact.gif

Note: I merely linked to the frames because I realize that viewing them may upset some people.

[genefromjersey]

Well, you can see the flames are burning hydrocarbons.(They've been speeded up - probably by a factor of 4 or 5-to make it look like a TNT/dynamite type explosion.)