In this paper, several numerical simulations of framed structures were performed to identify the specific structural cause of the high-speed total collapse of the World Trade Center (WTC) towers, which occurred during the 9/11 terrorist attacks. A full-scale aircraft impact simulation of the WTC tower 2 was conducted to examine the dynamic unloading phenomena that occurred in the core columns during impact, which may have caused the destruction of the splices between column sections. Fire-induced progressive collapse analyses of a high-rise tower with an outrigger truss system were carried out to qualitatively demonstrate the effects of fire patterns and structural parameters on the behavior of this towerfs collapse. In general, the tower remained standing for a longer period of time due to the catenary action of the outrigger truss system only if the load paths in the tower were protected and if the member connections were strong enough. However, in these analyses, the collapse speed never reached a value as high as that of the free fall observed in the WTC collapse, which occurred while the splices between column sections still retained their tensile strength. From these results, it is evident that the high-speed collapse of the WTC towers might have been caused by an inherent weakness in their member connections in addition to the destruction directly caused by the aircraft impact.