At 9:03 a.m. EDT on September 11 , 2001 , five terrorists crashed United Airlines Flight 175 into the southern facade of the South Tower . Three buildings in the World Trade Center complex , including 2 WTC , collapsed due to fire-induced structural failure . The light construction and hollow nature of the structures allowed the jet fuel to penetrate far inside the towers , igniting many large fires simultaneously over a wide area of the impacted floors . The fuel from the planes burned at most for a few minutes , but the contents of the buildings burned over the next hour to hour and a half . The fires might not have been as centrally positioned , nor as intense , had traditionally heavy high-rise construction been standing in the way of the aircraft . Debris and fuel would likely have remained mostly outside the buildings or concentrated in more peripheral areas away from the building cores , which would then not have become unique failure points . In this scenario , the towers might have stood far longer , perhaps indefinitely . The fires were hot enough to weaken the columns and cause floors to sag , pulling perimeter columns inward and reducing their ability to support the mass of the building above . The South Tower collapsed at 9:59 a.m. after burning for 56 minutes in the fire caused by the impact of United Airlines Flight 175 and the explosion of its fuel
United Airlines Flight 175
United Airlines Flight 175
2 World Trade Center