The American space agency Nasa says it has lost contact with the space shuttle Columbia about 15 minutes before it was due to land at the Kennedy space centre.
There were seven crew members aboard the shuttle - six Americans and the first Israeli in space.
Contact was lost over Texas
A Nasa spokesman confirmed they had had reports that the space shuttle had broken up during its descent, and that debris had fallen about 100 miles (160 km) south of Dallas, in the state of Texas.
Relatives of the astronauts who were awaiting the shuttle's arrival were taken away from mission control.
President Bush has been briefed about the incident and is monitoring the situation, a White House official said.
He is expected to make a statement shortly.
The White House has said there is no immediate evidence that terrorism was involved.
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In Israel, the office of Prime Minister Ariel Sharon issued a statement saying "the government and people of Israel are praying for the safety of the astronauts on board space shuttle Columbia."
Columbia, which was due to land in Florida at 0916 (1416GMT) was returning from a 16-day mission orbiting the Earth and had just begun its re-entry procedure when contact was lost.
If you find debris, please do not touch it, and if you have photos or videos that you think will be helpful, please contact your local police authorities
Nasa website
Television pictures showed a vapour trail from the craft as it flew over Dallas.
It then appeared to disintegrate into several separate vapour trails, and witnesses in the area said they heard loud noises.
One witness in the Dallas area, John Ferolito, told the BBC his house shook and windows rattled as he heard a loud boom.
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Nasa has declared an emergency, and search and rescue teams have been mobilised.
'Hazardous'
According to Nasa, the shuttle was about 200,000 feet up and travelling at 12,500 mph (20,000 kph) when contact was lost at about 0900 local time.
SHUTTLE CREW
Commander Rick Husband, US
Pilot William McCool, US
Kalpana Chawla, US
Laurel Clark, US
Ilan Ramon, Israel
David Brown, US
Michael Anderson, US
Although the space agency did not say what had happened to the shuttle, it warned that any debris found in the area should be avoided and could be hazardous.
In 42 years of human space flight, Nasa has never lost a space crew during landing.
In 1986, the space shuttle Challenger exploded shortly after lift-off with the loss of all seven crew on board.
Nasa spokesman John Ira Petty said that, having lost communication, he did not see how the shuttle could land.
He told the BBC: "It is controlled by computer until the last bit of the flight and is essentially a glider.
'Catastrophic failure'
"It doesn't have the capability to go around like a commercial airplane might."
Mr Petty said everyone at Nasa was very concerned about Columbia - Nasa's oldest shuttle, which first flew in 1981.
He added: "We are concerned about our crew members and their families but it is a situation where we have very little information."
Mr Petty said he could not think of a more difficult terrorist target than a shuttle, although he admitted security - particularly before launch - had been tighter than usual.
Former chief historian for Nasa, Roger Launius told BBC News that so far it looked like this was a "catastrophic failure."
