But, said Jones, molten aluminum is silvery. It never turns yellow. The substance observed in the videos "just isn't aluminum," he said. But, he said, thermite can cause steel to melt and become yellowish.

Second, he cited video pictures showing white ash rising from the south tower near the dripping, liquefied metal. When thermite burns, Jones said, it releases aluminum-oxide ash. The presence of both yellow-white molten iron and aluminum oxide ash "are signature characteristics of a thermite reaction," he said.

Another item of evidence, Jones said, is the fact that sulfur traces were found in structural steel recovered from the Trade Towers. Jones quoted the New York Times as saying sulfidization in the recovered steel was "perhaps the deepest mystery uncovered in the (official) investigation." But, he said, sulfidization fits the theory that sulfur was combined with thermite to make the thermite burn even hotter than it ordinarily would.

Jones said a piece of building wreckage had a gray substance on the outside that at one point had obviously been a dripping molten metal or liquid. He said that after thermite turns steel or iron into a molten form, and the metal hardens, it is gray.

He added that pools of molten metal were found beneath both trade towers and the 47-story WTC 7. That fact, he said, was never discussed in official investigation reports.

And even though WTC 7 was not connected to the Trade Towers — in fact, there was another building between it and the towers —and even though it was never hit by a plane, it collapsed. That suggests, he said, that it came down because a thermite fire caused its structural steel to fail.

Jones said his studies are confined to physical causes of the collapses, and he doesn't like to speculate about who might have entered the buildings and placed thermite and sulfur. But he said 10 to 20 people "in the know," plus other people who didn't know what they were doing but did what they were told, could have placed incendiary packages over several weeks.