The space shuttle Columbia was making a turn in its final seconds before it disintegrated in flames, NASA said.
The shuttle broke up as it was travelling 61,000 metres over eastern Texas about 9am local time (2am Sunday), moving at about 20,000kph.
At that point the mission controller in Houston, Texas communicating with the shuttle team, addressed Columbia commander Rick Husband.
"To Columbia, here is Houston; we see your tyre pressure messages and we did not copy your last" message.
After a moment, Husband replied: "Roger but ..."
After a brief crackling noise, contact was lost.
Columbia disappeared from radar screens 16 minutes before it was due to land. Several white trails of smoke were seen on the ground coming from bits of the shuttle.
All seven astronauts on board were killed in the break-up of Columbia.
NASA lost contact with the shuttle, which was completing a 16-day mission, while it was 63,000 metres above Earth, and 16 minutes from its scheduled landing at Kennedy Space Center.
Television images showed Columbia appeared to explode and break up above Texas, leaving several white trails across the blue skies instead of the single plume on a normal flight.
Officials in Nacogdoches, Texas, said residents reported spotting many pieces of the toxic debris dispersed throughout the college city of about 30,000 people, located about 230 km northeast of Houston.
Eyewitnesses say the shuttle appeared to "shed material" as it flew across the United States.
NASA is ruling out terrorism as the cause. The administrator of the NASA space agency dismissed rumours that the space shuttle tragedy could be the result of terrorism.
Sean O'Keefe addressed a news conference four hours after the shuttle disintegrated.
Commentators say no missile is capable of reaching the height at which Colombia began disintegrating.
Dangerous wreckage in Texas
Debris from Columbia rained down into parts of Texas with residents coming across pieces in fields and on roads, including what appeared to be a door from the orbiter, local officials and eyewitnesses said.
One piece of debris about one metre by 1.6 metres was smouldering in a field near Rice, Texas, just off Interstate 45 about 70 km south of Dallas.
Police were urging vehicles that slowed to look at the site to keep moving along the major highway that links Houston and Dallas.
"We do have a debris field. It is scattered all throughout Nacogdoches," city manager Victoria Lafollett said.
"What we've done is activated our emergency operations centre. Because we have so many pieces throughout the city, we're asking people to stay away from them. We're working closely with NASA and the FBI," she said.
The debris ranged in size and the city had received one report of a door from the shuttle being found, she added.
"The number of pieces being reported is just impossible to keep up with, she said. "Some are very small, some are larger."
NASA scrambled rescue units to search for wreckage and signs of the crew, which included the first Israeli to fly on the shuttle, former combat pilot Colonel Ilan Ramon. The rest of the crew consisted of six Americans, four men and two women.
Bush administration officials said there was no indication the breakup was due to terrorism.
However a White House spokesman said Homeland Security Secretary Tom Ridge had spoken to Texas Governor Rick Perry and homeland security officials in Louisiana and Oklahoma because debris was also believed to have rained down into those states.
The US space agency warned local residents to stay clear of the smouldering debris.
"Any debris that is located in the Dallas-Fort Worth vicinity should be avoided and may be hazardous due to the toxic nature of propellants used on board the shuttle and should be reported to local law enforcement authorities," NASA mission control in Houston said.
President addresses mourning nation
President George W Bush on Saturday led America in mourning the death of the seven astronauts aboard the doomed space shuttle, calling it a day of "great sadness" for their families and the entire nation.
"The Columbia is lost. There are no survivors," Bush said in a sombre address to the nation. "This day has brought terrible news and great sadness to our country."
Bush raced back to the White House from the Camp David presidential retreat soon after getting the tragic news.
He called the families of the astronauts who perished, comforting them in "this sudden shock and grief."
"You're not alone. Our entire nation grieves with you," Bush said as flags at the White House were lowered to half-staff.
Bush also called Israeli Prime Minister Ariel Sharon to express condolences over the loss of Israel's first astronaut, Ilan Ramon, who was flying on the shuttle when it broke up over Texas on its return from a 16-day science mission.
"These men and women assumed great risk in the service to all humanity in an age when space flight has come to seem almost routine," Bush said.
"Because of their courage and daring and idealism we will miss them all the more."
Administration officials said there was no immediate indication terrorism was involved - it was out of surface-to-air missile range - and no threat had been received targeting the spacecraft.
"There's no reason to believe there are any links to terrorism at this point, but we are fully investigating the situation," said a White House official, speaking on condition of anonymity.
But the Columbia disaster raised a new challenge for Bush as he prepared for a possible war with Iraq and searched for a peaceful solution to a nuclear standoff with North Korea.
Bush vowed the US space programme would continue after only the second loss of a space shuttle after Challenger blew up on takeoff in 1986. Columbia was NASA's oldest shuttle and first flew in 1981.
"The cause in which they died will continue," Bush said. "Our journey into space will go on."
© Reuters
