A Brigham Young University physicist said he now believes an incendiary substance called thermite, bolstered by sulfur, was used to generate exceptionally hot fires at the World Trade Center on 9/11, causing the structural steel to fail and the buildings to collapse.

"It looks like thermite with sulfur added, which really is a very clever idea," Steven Jones, professor of physics at BYU, told a meeting of the Utah Academy of Science, Arts and Letters at Snow College Friday.

The government requires standard explosives to contain tag elements enabling them to be traced back to their manufacturers. But no tags are required in aluminum and iron oxide, the materials used to make thermite, he said. Nor, he said, are tags required in sulfur.

Jones is co-chairman, with James H. Fetzer, a distinguished professor of philosophy at the University of Minnesota of Scholars for 9/11 Truth, a group of college faculty members who believe conspirators other than pilots of the planes were directly involved in bringing down New York's Trade Towers.

The group, which Jones said has 200 members, maintains a Web site at www.st911.org. A 40-page paper by Jones, along with other peer-reviewed and non-reviewed academic papers, are posted on the site.

Last year, Jones presented various arguments for his theory that explosives or incendiary devices were planted in the Trade Towers, and in WTC 7, a smaller building in the Trade Center complex, and that those materials, not planes crashing into the buildings, caused the buildings to collapse.

At that time, he mentioned thermite as the possible explosive or incendiary agent. But Friday, he said he is increasingly convinced that thermite and sulfur were the root causes of the 9/11 disaster.

He told college professors and graduate students from throughout Utah gathered for the academy meeting that while almost no fire, even one ignited by jet fuel, can cause structural steel to fail, the combination of thermite and sulfur "slices through steel like a hot knife through butter."

He ticked off several pieces of evidence for his thermite fire theory:

First, he said, video showed a yellow, molten substance splashing off the side of the south Trade Tower about 50 minutes after an airplane hit it and a few minutes before it collapsed. Government investigators ruled out the possibility of melting steel being the source of the material because of the unlikelihood of steel melting. The investigators said the molten material must have been aluminum from the plane.

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.