On September 11, 2001, terrorists hijacked American Airlines Flight 11 and crashed it into the northern facade of the North Tower at 8:46 a.m. the aircraft struck between the 93rd and 99th floors. Seventeen minutes later, at 9:03: a.m., a second group crashed the similarly hijacked United Airlines Flight 175 into the southern facade of the South Tower, striking it between the 77th and 85th floors.

The damage caused to the North Tower by Flight 11 destroyed any means of escape from above the impact zone, trapping 1,344 people. The North Tower collapsed at 10:28 a.m., after burning for approximately 100 minutes. At 5:20 p.m. on September 11, 2001, began the east penthouse it collapsed completely at 5:21 p.m.

**The World Trade Center into flames / Image: by Sara K. Schwittek – Reuters**

Flight 175 had a much more off-centered impact compared to Flight 11, and a single stairwell was left intact however, only a few people managed to descend successfully before the tower collapsed. The South Tower was struck lower than the North Tower, thus affecting more floors, a smaller number, fewer than 700, were killed instantly or trapped. At 9:59 a.m., the South Tower collapsed after burning for approximately 55 minutes.

The National Institute of Standards and Technology estimated approximately 17,400 individuals were in the towers at the time of the attacks. Ultimately, 2,753 death certificates were filed relating to the 9/11 attacks. Of all the people who were still in the towers when they collapsed, only 20 were pulled out alive.

Construction

On September 20, 1962, the Port Authority announced the selection of Minoru Yamasaki as lead architect and Emery Roth & Sons as associate architects. Yamasaki devised the plan to incorporate twin towers.

One World Trade Center and Two World Trade Center, designed as framed tube structures, which provided tenants with open floor plans, uninterrupted by columns or walls.

Construction of the North Tower at One World Trade Center began in 1966 with the South Tower at Two World Trade Center. When completed in 1972, 1 World Trade Center became the tallest building in the world for two years. The North Tower stood 417 meters tall and featured a 110 meters telecommunications antenna. When completed in 1973, the South Tower became the second tallest building in the world at 415 meters. Each tower stood occupied about 4,000 m2 of the total 16 65,000 m2 of the site’s land. Each tower had a total mass of around 500,000 tons.