The twin towers of the World Trade Center collapsed on September 11, 2001, as a result of al-Qaeda's September 11 attacks, in which terrorists affiliated with al-Qaeda hijacked four commercial passenger jet airliners, flying one into the North Tower (1 World Trade Center) and another into the South Tower (2 World Trade Center). The South Tower collapsed at 9:59 a.m., less than an hour after being hit by the hijacked airliner, and at 10:28 a.m. the North Tower collapsed. Later that day, 7 World Trade Center collapsed at 5:21 p.m. from fires that had started when the north tower collapsed. As a result of the attacks to the towers, 2,752 people died, including all 157 passengers and crew aboard the two airplanes. The collapse of the twin towers also caused extensive damage to the rest of the complex and nearby buildings. Debris from the collapsing towers severely damaged or destroyed more than a dozen other adjacent and nearby structures.
The Federal Emergency Management Agency (FEMA) completed its performance study of the buildings in May 2002. It declared that the WTC design had been sound, and attributed the collapses wholly to extraordinary factors beyond the control of the builders. While calling for further study, FEMA suggested that the collapses were probably initiated by weakening of the floor joists by the fires that resulted from the aircraft impacts. According to FEMA's report, the floors detached from the main structure of the building and fell onto each other, initiating a progressive "pancake" collapse.
FEMA's early investigation was revised by a later, more detailed investigation conducted by the National Institute of Standards and Technology (NIST), which also consulted outside engineering entities. This investigation was completed in September 2005. Like FEMA, NIST vindicated the design of the WTC, noting that the severity of the attacks and the magnitude of the destruction was beyond anything experienced in U.S. cities in the past. NIST also emphasized the role of the fires, but it did not attribute the collapses to failing floor joists. Instead, NIST found that sagging floors pulled inward on the perimeter columns: "This led to the inward bowing of the perimeter columns and failure of the south face of WTC 1 and the east face of WTC 2, initiating the collapse of each of the towers."
The cleanup of the site involved round-the-clock operations, many contractors and subcontractors, and cost hundreds of millions of dollars. The demolition of the surrounding damaged buildings continued even as new construction proceeded on the World Trade Center's replacement, 1 World Trade Center , which will top out in late spring or early summer 2012. Only 7 World Trade Center has been replaced as of 2012.