Force of the impact and the resulting fire from aviation fuel destroyed most elevators and most staircases between the floors above the hundredth and below the ninety-third floors. Nearly one thousand people were trapped on the upper floors of the North Tower. A majority of them worked for Cantor Fitzgerald brokerage company. At least sixty people jumped from the North Tower rather than burn to death. One firefighter was killed after being hit by one of the jumpers.

An emergency call went out to the Fire Department of New York City (FDNY). More than a thousand firefighters from 225 units showed up at the World Trade complex. There were so many vehicles that parking became a problem. Immediately, FDNY commanders realized that they could not extinguish the growing fire in the North Tower, so they concentrated on evacuating people. Because of lack of water, only a few firefighters would engage in trying to put out the fire. Operators for the 911 system told people to stay put, and assured them that firefighters would be coming to rescue them. To those on the top floors of the North Tower the deteriorating conditions made it imperative that help come soon. Some tried to make it to the roof, but the FDNY had decided after the 1993 World Trade Center bombing to lock the heavy doors leading from the floors to the building's sole roof exit. This decision had been made because rooftop rescues by helicopters were a safety risk.

Events at the North Tower caused concern among those in the South Tower. Many of those in the South Tower decided to evacuate the building. Those who tried to evacuate the South Tower were told as they tried to leave the building to return to their offices. This was because the standard firefighting philosophy in high-rise fires was to “stay put, stand by.” An announcement broadcast over the intercom at 8:55 a.m. stated that there was no need to evacuate the South Tower. This announcement directly contradicted a decision by Sergeant Al DeVona, ranking Port Authority police officer on the scene, who had ordered that both the North and South Towers be evacuated within minutes of the first crash. DeVona reordered the evacuations at 8:59 a.m. Captain Anthony Whitaker, commander of the Port Authority Police, confirmed this order shortly thereafter. Faulty communications equipment made these decisions difficult to implement. Because of the communication problems, the 911 operators could not be informed of the deteriorating situation and they continued to give outdated advice to people to stay where they were.

The difficulty in evacuating the twin towers was compounded by the structural defects of the towers. Decisions made during construction made it difficult for people to evacuate, there being only three staircases. Changes in building codes in 1968 had reduced both the number of staircases and the level of fire protection for high-rise buildings. These changes allowed more rentable space, but meant that the staircases were built for only a few hundred people at a time to walk three or four stories, not for mass evacuation. The location of the three staircases in the center of the building, rather than being dispersed, turned the upper floors of both towers into death traps because the plane crashes in both buildings cut off access to the staircases. With inferior fireproofing, there was nothing to prevent the spread of the fires. The New York City building codes were not an issue because the builder, the Port Authority of New York and New Jersey, as a regional entity was not required to follow building codes.

The twin towers had been able to sustain the impact of the two airliners, but it was the fire that endangered the structure. Later reports indicated that it was the high temperature of the fire, caused by the ignition of the aviation fuel and intensified by the burning office furniture and paper, that caused the worst damage. Another factor was that the World Trade Center complex buildings had been constructed with thirty-seven pounds of steel per square foot, in contrast to the normal high-rise buildings of that era, which were built with seventy-five pounds of steel per square feet. This type of construction saved millions of dollars during construction and increased the square footage of rental space, making the buildings more profitable. Since steel begins to degrade at 300 degrees and continues to degrade by 50 percent at 1,000 degrees, the high-temperature fire, combined with the reduced amount of steel supporting the buildings, led to weakening of the structure of the buildings.

The South Tower collapsed first. United Airlines Flight 175 hit at higher speed than did American Airlines Flight 11. This higher speed and the resulting explosion and fire in the South Tower caused it to collapse in a heap.

Two firefighters hose down a hot spot as another looks on from above at the site of the destroyed World Trade Center, October 9, 2001. (AP IMAGES/Stuart Ramson.)

Total deaths at the World Trade Center were 2,749. Of this total, 147 were passengers and crew of the two aircraft. Another 412 of the dead were rescue workers killed when the two towers collapsed. The remaining 2,190 dead succumbed to the plane crashes or in the collapse of the towers. Except for the actions of key individuals and the firefighters, the casualties could have been much higher. Except in the case of those trapped in the North and South Towers above where the planes hit, there was no discernable reason for some people to have died while others survived.

Besides attacking the physical structures of the twin towers, the hijackers also impacted the financial health of the United States. Many businesses simply went out of business in New York City. Many others struggled to regain financial viability. Layoffs in the period between September 12 and January 21, 2002, related to September 11 were calculated at 1,054,653. The attack on the airline industry was an indirect financial blow in that it accounted for about 9 percent of the total gross domestic product of the United States, and because around eleven million jobs are related directly to commercial aviation.