Genady P. Cherepanov, Ivan E. Esparragoza TWO VERSIONS OF WTC COLLAPSE There are two basic versions of the collapses of the World Trade Center towers in New York on September 11, 2001. <...> According to the official version of National Institute of Standards that studied the collapses on an assignment of the President and government of the United States, all collapses represented the progressive collapse by floor-by-floor failure started from a critical floor, which was assumed to be the floor hit by terrorists and subjected to fire. <...> According to the other version widely-spread in the United States, MASAD agents mined a floor below those under fire, and detonated explosives in a while after the crushes. <...> In the present paper, the official version of U.S. government is assumed valid, and the critical floor where the collapses started from is calculated based on: (i) the governing equations of progressive collapse formulated by Cherepanov in Int. <...> It is proven that all collapses started from a floor located significantly lower than those hit by terrorists and subjected to fire. <...> Keywords: Progressive collapse, WTC, two versions, critical floor, governing equations, free fall . 1. <...> INTRODUCTION On an assignment of the President and government of the United States, U.S. National Institute of Standards, together with prominent mechanical engineers of the U.S. National Academy of Sciences, U.S. National Academy of Engineering and British Royal Society, studied the collapses of the World Trade Center towers that occurred in New York on September 11, 2001. <...> The official version they worked out said that all collapses represented the progressive, floor-byfloor collapse started from a critical floor that was assumed to be the floor hit by terrorists and subjected to fire. <...> Simultaneously, there existed another version widely-spread in the United States, that MASAD agents mined a floor below those under fire and detonated explosives in about one hour after the crushes. <...> The free fall time of all collapses including the neighboring 47-story building was welldocumented. <...> The above-mentioned authorities of engineering believed that this fact is explained within the framework of progressive collapse by almost zero resistance of the underlying, intact structure to the falling down, upper structure. <...> J. Fracture (2006), 141:287-289 where it has been proven that, even for zero resistance of the underlying structure, the acceleration of the falling, upper structure is significantly smaller than the gravitation acceleration <...>