Up to 800 people may have died when a hijacked plane crashed into the Pentagon, according to officials.
Washington hospitals have reported 71 people injured while between 100 and 800 are listed as missing and possibly dead.
An American Airlines flight from Washington DC to LA crashed into the Pentagon with 64 passengers and crew on board.
Secretary of Defence Donald Rumsfeld said: "There are a number of people that they have not identified by name, but have identified as being dead, and there are a number of casualties.
"The number has yet to be calculated, and it will not be a few."
The Pentagon suffered widespread damage on its the building's fourth, fifth and sixth corridors. The impact tore a massive hole in one side of the building.
The jet struck a section of the building that housed US Army offices.
Eyewitness Mike Walter told CNN: "It was like a cruise missile with wings, went right there and slammed into the Pentagon."