On the morning of September 11, 2001, four hijacked American Airline planes flying out of Boston were deliberately crashed into the two World Trade Center Towers, the Pentagon in Washington DC, and a field in rural Pennsylvania. A total of approximately 3,000 persons died in the attacks.
The real estate damages alone were staggering, being estimated by New York City's Mayor's Office as:
There was initial discussions of making the World Trade Center site a memorial, however, the economics of this did not make sense. A more balanced approach was adopted that would dedicate a portion of the site as a memorial and also allow rebuilding of office buildings.