Firefighters work at Ground Zero, the site of the World Trade Center attacks, on Sept. 11, 2001.

Firefighters work at Ground Zero, the site of the World Trade Center attacks, on Sept. 11, 2001.

Firefighters work at Ground Zero, the site of the World Trade Center attacks, on Sept. 11, 2001.

Firefighters work at Ground Zero, the site of the World Trade Center attacks, on Sept. 11, 2001.