The recording also captured the sound of a struggle in the cockpit as the four attackers took control of the plane, and conversations between the aircraft and an air traffic controller in Cleveland.

All 44 passengers and crew on board the Newark-San Francisco flight died when the plane was brought down in a field after passengers struggled with the four hijackers.

The tape, broadcast on ABC television, reveals a series of routine communications between the cockpit and air traffic controllers, interrupted by the sound of struggling and voices shouting.

Screaming can be heard, followed by an American voice shouting "get out of here".

Later, one of the hijackers is heard telling passengers to remain seated and that there is a bomb on board.

Later still, a second hijacker delivers a similar message over the aircraft's intercom.

"Hi. This is the captain, I would like you all to remain seated. There is a bomb on board and we are going back to the airport. They have our demands..." the hijacker says.

At this point the air traffic controller in Cleveland asks another nearby aircraft to confirm what he has just heard.

"Did you understand that transmission?" the controller asks.

"Affirmative. He said there was a bomb on board," the other aircraft replies.

The air traffic controller then asks: "United 93, please, do you still hear the centre?"

The controller tries unsuccessfully to make contact with the plane no fewer than 20 times.

UA Flight 93, the fourth aircraft to be hijacked on 11 September, crashed into a field in Shanksville, Pennsylvania before the hijackers could fly it to its final, unknown destination.

Some of the passengers are reported to have called relatives on their mobile phones and told them they were planning to tackle the hijackers.

Investigators said transcripts of the cockpit voice recording reveal three passengers tried to overpower the men, but were unable to keep control of the plane.

http://news.bbc.co.uk/hi/english/world/americas/newsid_1659000/1659401.stm