Two commercial airliners hijacked by terrorists were deliberately flown into the Twin Towers of the complex, resulting in a total progressive collapse that killed almost 3,000 people.

The North Tower (WTC 1) was the first building to be hit when American Airlines Flight 11 crashed into it at 8:46 a.m.,[a] causing it to collapse at 10:28[e] after burning for one hour and 42 minutes.

The towers' destruction caused major devastation throughout Lower Manhattan, and more than a dozen adjacent and nearby structures were damaged or destroyed by debris from the plane impacts or the collapses.

Toxic powder from the destroyed high-rises was dispersed throughout the city and gave rise to numerous long-term health effects that continue to plague many who were in the towers' vicinity, with at least three additional deaths reported.

Demolition of the surrounding damaged buildings continued even as new construction proceeded on the Twin Towers' replacement, the new One World Trade Center, which opened in 2014.

[20] Built with a novel "framed tube" design that maximized interior space, the towers had a high strength-to-weight ratio requiring 40 percent less steel than more traditional steel-framed skyscrapers.

The floors were connected to the perimeter spandrel plates with viscoelastic dampers, which helped reduce the amount of sway felt by building occupants.

[26] Leslie Robertson, one of the chief engineers working on the design of the World Trade Center, said that he considered the scenario of the impact of a Boeing 707, which might be lost in the fog and flying at relatively low speeds while seeking to land at either JFK or Newark Airports.

He also said that the main difference between the design studies and the event that caused the towers to collapse was the velocity of the impact, which greatly increased the absorbed energy, and was never considered during the construction process.

[28] During its investigation into the collapse, the National Institute of Standards and Technology (NIST) obtained a three-page white paper that stated the buildings would survive an aircraft-impact of a Boeing 707 or DC 8 flying at 600 mph (970 km/h).

[29] In 1993, John Skilling, lead structural engineer for the WTC, said in an interview conducted after the 1993 World Trade Center bombing: "Our analysis indicated the biggest problem would be the fact that all the fuel would dump into the building.

But in April 1970, the New York City Department of Air Resources ordered contractors building the World Trade Center to stop the spraying of asbestos as an insulating material[33] and vermiculite plaster was used instead.

In its final moments, American Airlines Flight 11 flew south over Manhattan and crashed at roughly 440 mph (710 km/h) into the northern facade of the North Tower (WTC 1) at 8:46 a.m., hitting the 93rd through 99th floors.

[37] Seventeen minutes later, United Airlines Flight 175 flew northeast, over New York Harbor, and crashed into the southern facade of the South Tower (WTC 2) at 9:03 a.m.,[g] striking between the 77th through 85th floors at 540 mph (870 km/h).

[43] The towers' light construction and hollowness allowed the jet fuel to penetrate far inside them, igniting many large fires simultaneously over a wide area of the impacted floors.

Three were spotted from an east-facing window on the south side of the 79th floor,[49] while the 100–200 people who fell or jumped from the four faces of the North Tower had no other means of escape from the insufferable heat, smoke and fire consuming its top 18 stories.

The damage to the North Tower by Flight 11's centered impact severed all escape routes above the 91st floor and left the stranded workers in an insufferable inferno from which jumping was their only means of escape; Flight 175 struck the South Tower through the southeast corner of the skyscraper's southern facade and left the northwesternmost stairwell undamaged from top to bottom.

As the collapse progressed, dust and debris could be seen shooting out of the windows several floors below the advancing destruction, caused by the sudden rush of air compressed under the descending upper levels.

This was because Flight 11 struck more or less in the center, causing more symmetrical impact damage to the North Tower's core and leaving more of its structural support intact.

[71][69] The implied threat of an imminent collapse was sufficient for the NYPD to order its officers to evacuate, although none of the helicopter pilots specifically predicted that either tower would fall.

As the North Tower collapsed, heavy debris hit 7 World Trade Center, causing damage to the south face of the building[79] and starting fires that continued to burn throughout the afternoon.

[86] At approximately 2:00 pm, firefighters noticed a bulge in the southwest corner of 7 World Trade Center between the 10th and 13th floors, a sign that the building was unstable and might cave to one side or "collapse".

In the immediate aftermath of the attacks, numerous structural engineers and experts spoke to the media, describing what they thought caused the towers to collapse.

[101] César Pelli, who designed the Petronas Towers in Malaysia and the World Financial Center in New York, remarked, "no building is prepared for this kind of stress.

"[102] On September 13, 2001, Zdeněk P. Bažant, professor of civil engineering and materials science at Northwestern University, circulated a draft paper with results of a simple analysis of the World Trade Center collapse.

Bažant suggested that heat from the fires was a key factor, causing steel columns in both the core and the perimeter to weaken and experience deformation before losing their carrying capacity and buckling.

Only a fraction of a percent of the buildings remained for analysis after the cleanup was completed: some 236 individual pieces of steel, although 95% of structural beams and plates and 50% of the reinforcement bars were recovered.

[110] Congress passed the National Construction Safety Team bill in October 2002, giving NIST the authority to conduct an investigation of the World Trade Center collapses.

James Quintiere, professor of fire protection engineering at the University of Maryland, called the fact that only a small portion of the building's steel was preserved for study "a gross error" that NIST should have openly criticized.

[141] Significant long term medical and psychological effects have been found among first responders including elevated levels of asthma, sinusitis, gastroesophageal reflux disease and post-traumatic stress disorder.