On September 11 , the North Tower ( 1 WTC ) was hit by American Airlines Flight 11 and the South Tower ( 2 WTC ) was hit by United Airlines Flight 175 , both Boeing 767 aircraft . The South Tower collapsed 56 minutes after the impact , and the North Tower collapsed 102 minutes after . An investigation by NIST concluded that the collapse was caused by a combination of damage to support columns and fire insulation from the aircraft impacts and the weakening of columns and floors by jet fuel ignited fires . NIST also found " no corroborating evidence for alternative hypotheses suggesting that the WTC towers were brought down by controlled demolition using explosives planted prior to September 11 , 2001 "
United Airlines Flight 175
United Airlines Flight 175
World Trade Center controlled demolition conspiracy theories