World Trade Center's signature Twin Towers drew the world's attention during the 9/11 attacks when the Islamic terrorist group al-Qaeda hijacked two Boeing 767 jets and rashed them into the North and South Towers within minutes of each other. The towers both collapsed within hours, taking near 3000 lives, making it the single deadliest terrorist attack in human history. The construction work for the World Trade Center started on August 5, 1966. The complex was opened on April 4, 1973. The Twin Towers, the original 1 World Trade Center, at 1,368 feet (417 m); and 2 World Trade Center, at 1,362 feet (415.1 m), were the tallest buildings in the world at the time of their completion.