The Twin Towers construction began in 1966 at a cost of $1.5 billion. By 1970, one tower was ready for occupants, though upper floors were unfinished. Construction completed in 1973, finishing the upper floors and making the World Trade Center a reality. On September 11, 2001, two planes hijacked by Al Qaeda terrorists crashed into the towers, resulting in their collapse and killing nearly 3000 people. It was an unprecedented act of terrorism that changed America.