Although some official statements have been released by the Federal Emergency Management Agency (FEMA) and the National Institute of Standards and Technology (NIST), the 9/11 tragedy of the New York World Trade Center (WTC) towers still remains as an unresolved mystery since the towers collapsed totally to the ground, and, in an unnaturally rapid speed. We applied a low-cost, highly accurate collapse analysis code using the ASI-Gauss technique to several numerical simulations of framed structures, to seek the true cause of the highly-rapid total collapse. One of its applications is a full model simulation of the WTC 2 subjected under an aircraft impact. According to the simulation, gspringbackh phenomena due to rapid unloading were observed in the core columns during the impact, which might have caused the destruction of member joints. Another simulation carried out was that of the fire-induced collapse of a high-rise tower. The results clearly show the effect of the weak member joints, which were reported to be 20 to 30 % of the member strengths in the WTC towers, and also the effect of the strength reduction due to elevated temperatures. It is possible that the highly-rapid total collapse of the WTC towers was caused not only by the buckling and strength reduction of members due to elevated temperatures, but by the original weakness of the member joints and the destruction of those during the collision of the aircraft.