G.P. Cherepanov, I.E. Esparragoza WHAT INITIATED COLLAPSES IN NEW YORK ON SEPTEMBER 11, 2001? <...> A new, more sophisticated analysis of the WTC collapse called the hybrid theory is advanced below that uses both fracture waves and progressive failure. <...> According to the hybrid theory the collapses of World Trade Center towers ran in two phases: (i) in the first phase dynamic fracture waves disintegrated a considerable part of the towers producing the dust cloud and explosion sound well-documented and well-evidenced, and (ii) in the second phase the progressive failure front disintegrated the lower part of the towers that remained intact in the first phase. <...> To illustrate many possible modes of progressive failure the slowest mode, the fastest mode, and three intermediate modes were first studied and then the hybrid mode was introduced. <...> The hybrid theory allows one to get rid of evident defects of earlier theories and explain all basic facts and observations of the matter not understood before, in particular, why the time of all collapses was free fall time independent of the position of critical floor the collapses started from. <...> It was shown that the floors disintegrated in the very beginning of the collapses were located considerably lower than the floors hit by terrorists and subjected to fire. <...> From here, it follows that fracture waves were produced by explosives and not arisen from thermal stresses of fire as suggested earlier. <...> Keywords: Collapse, tower, WTC, mechanics, theory, evidence, observation, dust cloud, explosion, free fall, critical floor, fracture wave, progressive failure, the slowest mode, the fastest mode, intermediate mode, hybrid mode, analysis, calculations 1.INTRODUCTION Progressive failure was the common mode of all collapses of tall buildings as emphasized by Bazant and Verdure (2006). <...> They believed the WTC collapses represented also a simple progressive failure caused by the following chain of bad events: the crush of terrorist plane into a critical floor set a fire, severed 13% of the total of 287 steel columns and supposedly stripped fire insulation from all columns that supposedly collapsed in one hour from creep buckling, the start of general progressive collapse by buckling floor-by-floor up to the ground. <...> They think that was established by NIST <...>