Scientists, Engineers, and Architects analyze the physical evidence:
The main facts:
1. Some of the steel structural columns and girders of the World Trade Center towers, weighing many tons, were blasted sideways with such velocity that they stuck in the facades of other buildings hundreds of feet away. This video of the destruction of the North Tower shows large numbers of huge, multi-ton steel girders and columns -- the straight-line objects -- flying through the air:
http://www.youtube.com/watch?v=nUDoGuLpirc
It's impossible for a collapse caused only by fire and gravity (the government's "pancaking" lie) to have propelled those large, very heavy pieces of steel sideways like that. The force of gravity can only push objects downward. The probability of so many girders striking something or being struck by something in just such a way as to bounce them sideways with such speed is nil. The sideways force could only have come from explosives, which had to have been planted in the towers beforehand. Arab terrorists could not have gotten into the towers to do that.
2. As the towers were destroyed, the line of disintegration descended at two-thirds the rate of a free-falling object. If the cause of destruction had been "pancaking" -- the weight of the rubble crushing each level in turn -- as the government says, each intact level would have resisted the fall of the debris reaching it and slowed it down, to a small fraction of the speed of a free-falling object. Therefore, each level must have been blasted away as the rubble reached it.
3. Below ground level, huge puddles of metal were found that stayed liquid for a month (due to the thermal insulation of the rubble on top of them). Jet fuel is kerosene -- like charcoal starter fluid. When burning it never reaches the melting point of steel. The steel was melted by thermite -- aluminum-powder incendiary -- enhanced with sulfur (which forms a lower melting-point compound with iron); the combination is called "thermate".
4. Particles of unburned thermite have been found in the dust from the demolition of the towers, and these have now been thoroughly analyzed by physical and chemical methods. They are very small (0.2 - 3.0 mm) and thin (0.02 - 0.2 mm) chips containing extremely small grains of aluminum and iron oxide, which could only have been made by high-tech processes. It is impossible for this nano-thermite material to have been created by Osama bin Laden and "Al-Qaeda". But it has been made by the U.S. government for various uses in weapons.
5. In 1990 a PBS video about the construction of the Twin Towers was briefly broadcast on TV. It showed that each tower had a huge reinforced concrete core at its center. (These cores were omitted from the structural diagrams furnished by the government after 9-11.) The 3-inch thick rebars in these cores were coated with a plastic substance, supposedly to protect them from corrosion. This substance was so flammable that special welders with security clearances had to be called in to weld the rebars as each three-storey segment of the buildings was added. In fact the coating was a plastic explosive similar to C-4. There were additional explosive charges placed in other key structural locations. These are what demolished the towers on 9-11, by snapping the vertical columns at their connections, and blowing up the central cores. The video has been covertly removed from all public libraries.
Conclusion: the World Trade Center towers were demolished above-ground by explosives and below-ground by incendiaries, planted in the buildings. It was impossible for Arabs to get in and do that. But the company that provided security for the WTC had Marvin Bush -- brother of George W Bush -- on its board of directors.
See the Nanothermite Research, 9/11 Research Sites, and Concrete Cores sections below for the research that has led to the above conclusions.
Nanothermite Research -- Proof that the government did 9/11!
Active Thermitic Material Discovered in Dust from the 9/11 World Trade Center Catastrophe by Steven Jones, et al (PDF)
http://www.benthamscience.com/open/tocpj/articles/V002/7TOCPJ.pdf