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It's also true that within the "9-11 movement" there are large numbers of people promoting silly or sinister theories, so much so that the more moderate elements have to spend a lot of time disavowing them, which is presumably the point. The more moderate elements, as I put it, concentrate on the mechanics of the collapses (of the Towers, as well as of Building 7) and seem to me to have made convincing engineering points. I am especially impressed by the work of Gordon Ross, who is himself a structural engineer. There is a 25-minute video of him on googlevideos explaining why the towers fell in the bizarrely spectacular way they did, which really doesn't make any sense at all on the conventional view:
http://video.google.com/videoplay?docid=4801566025292753615
and he covers the same material on his website at
http://gordonssite.com/index.html
The merit of this type of argument is that it doesn't require any hypothesis about "who" or "why", except the minimal obvious one, which is that whoever-it-was had access to the maintenance areas (such as the liftshafts) of the three WTC buildings for a few dozen hours over the weeks before the events.
One can, however, easily develop a circumstantial argument regarding motive, as Jerry Mazza does here:
http://onlinejournal.com/artman/publish/article_3135.shtml
Incidentally, from the psychological warfare point of view, the '9-11 Truthling' community is really interesting (the derisive term comes from the video troll who currently glories in the nickname of 'Nicointelpro Haupt').