Abstract: An argument is presented that all theories of progressive collapse of the Twin Towers fail because the initiating event could not have occurred. The NIST report appears to be unsatisfactory in that it fails to deal with certain observations. The NIST report of the tragic events of 9/11 includes a study of the factors which led to the Twin Towers being ‘poised for collapse’. The report did not go on to address the mechanism of the actual collapse of the towers. Readers are left with the assumption that, from that moment, collapse was inevitable. The report has been rightly criticized on the grounds that NIST, in omitting reference to the collapse mechanism, failed to perform the fundamental task it had been given.