19 al-Qaeda terrorist hijacked four passenger airlines which were under US passenger air carriers (United Airlines and American Airlines). Two of the planes, American Airlines Flight 11 and United Airlines Flight 175, loaded with 20,000 gallons of jet fuel crashed into the North and South towers, respectively, of the World Trade Centre complex in New York City.

Soon after the crash, the gigantic structures of 110 storeys came down crushing, resulting in millions of tonnes debris and huge fires which caused complete collapse of other buildings in the World Trade Centre complex, including the 47-storey 7 World Trade Centre tower, as well as significant damage to 10 other large surrounding structures.

About 18 minutes after the first plane hit and media reported the incident as an accident, a second Boeing 767–United Airlines Flight 175–appeared in the sky, turned sharply towards the World Trade Centre and rammed into the South Tower near the 60th floor.

The third plane crashed into the Pentagon (the headquarters of the United States Department of Defense) in Arlington County, Virginia, leading to a partial collapse of the building's western side.

The fourth plane, United Airlines Flight 93, initially was heading towards Washington, D.C., but crashed into a field in Stonycreek Township near Shanksville, Pennsylvania, after its passengers tried to overcome the hijackers. It was the deadliest incident for fire-fighters and law enforcement officers in the history of the United States, with 343 and 72 killed respectively.

(Image Credit: 9/11 Memorial)