"free fall" acceleration of 7 WTC during part of the collapse, to "lateral ejection of steel," and to "mid-air pulverization of concrete." Gage also said that the absence of "large gradual deformations" associated with the collapse would indicate that the buildings have been destroyed by controlled demolition. That the three buildings of the World Trade Center "fell through what should have been the path of greatest resistance" would, according to the organization, require "precisely timed removal of critical columns, which office fires cannot accomplish". As the mass of the top of the North Tower had been blown outward during the collapse, there was "nothing left to drive this building to the ground," Gage says. Gage maintains that the "sudden and spontaneous" collapse of the towers would have been impossible without a controlled demolition, that pools of molten iron found in the debris of the buildings were evidence of the existence of thermite, and that researchers had found unignited nano-thermite in the dust produced by the collapse of the World Trade Center. Gage argues that this material "is not made in a cave in Afghanistan". Iron-rich micro-spheres, which, according to the organization, have been found in the dust of the World Trade Center buildings by independent laboratory analyses, would indicate temperatures during the collapses much higher than temperatures that would result from hydrocarbon fires. A DVD produced by the group contains eyewitness accounts of claimed explosions and flashes seen in the buildings. In 2008, Zdeněk P. Bažant, professor of civil engineering and materials science at Northwestern University, published with three coauthors a paper to examine whether allegations of controlled demolition might be scientifically justifiable. They found that the available video records are not consistent with the free fall hypothesis, that the size of the concrete particles is consistent with comminution caused by impact, and that the high velocity of compressed air explains why material from the towers were ejected to a distance of several hundred meters from the tower. The authors conclude that the allegations of controlled demolition do not have any scientific merit. A spokesman for NIST said that any sightings of molten metal, including metal seen pouring from the South Tower, were likely molten aluminum from the airplane, an explanation disputed by Gage who stated that the color of the molten metal rules out aluminum. "Basically, gravity and the utter force of the upper floors forced the towers down,"