At the time of their completion, the original 1 World Trade Center (the North Tower) and 2 World Trade Center (the South Tower), known jointly as the "Twin Towers", were the tallest buildings in the world. On the morning of September 11, 2001, hijackers flew two Boeing 767 jets into the complex, one into each tower. After burning for 56 minutes, the South Tower (2) collapsed, followed a half-hour later by the North Tower (1). The attacks killed 2,753 people.