Richard Gage : Architects & Engineers for 9/11 Truth (english) 
Richard Gage worked for over 20 years as an architect and constructed several high-rise buildings during that time. He is the founder of "Architects and Engineers for 9/11 Truth" an organization of more than 1900 architects and engineers from allover the world who evaluated the available evidence related to 9/11 and came to the conclusion that the official version of the events on 9/11 does not add up from a physics point of view.
Since the group also has members who are material scientists these experts conducted tests of the four available dust samples collected on 9/11. In all 4 samples they found residues of a special form of explosive : Nano-Thermite
This kind of substance actually has no reason to be there except if you buy into the theory that the buildings were brought down by controlled demolition. The fact that World Trade Center 7 - a 47-level high-rise building - collapsed on the afternoon of 9/11 within seconds virtually at free-fall speed was hardly ever reported on mainstream media. All this causes more and more questions, e.g. the question why the collpse of WTC7 was not even mentioned in the 9/11 commission report which represents the current official history record.
This lecture was conducted in May of 2008 and represents the state of 9/11 research by Richard at that time :
Additional information :
In the Knowledge Base you find an article on 12 topics why there are severe issues regarding the "official story" on 9/11
The organization Architects and Engineers for 9/11 Truth has produced the documentary "Explosive Evidence - Experts speak out". It is available in the Resource section