"It's gone," said a senior U.S. official who spoke on condition of anonymity. The official said debris from the shuttle, spread across Texas and apparently some other states, had been positively identified. Though there was no official word from NASA, the official said there was no hope for either the shuttle or its crew.

Six Americans and the first Israeli astronaut were on board.

At Kennedy Space Center, the U.S. flag next to the countdown clock was lowered to half-staff. NASA officials wouldn't confirm that the crew was dead but said President Bush would be making an announcement. The flag over the White House was lowered to half-staff, as well.

A senior U.S. official, speaking on condition of anonymity, said that there was no immediate indication of terrorism and no specific threats had been made against Columbia. The shuttle would have been out of range of a surface-to-air missile at the time, the official said.

Columbia had been expected to land at Kennedy Space Center in Florida at 9:16 a.m.

At 9 a.m., Mission Control suddenly lost all data and voice contact with the shuttle and crew. At the same time, residents of Texas, Arkansas and Louisiana reported hearing "a big bang" and seeing flames in the sky.

Television footage showed a bright light followed by smoke plumes streaking diagonally through the sky. Debris appeared to break off into separate balls of light as it continued downward. NASA declared an emergency after losing contact with the crew and within minutes said search teams had been sent to the Dallas-Fort Worth area.

In Nacogdoches, Texas, residents found bits of metal strewn across the city. Dentist Jeff Hancock said a metal bracket about a foot long had crashed through his office roof.

"It's all over Nacogdoches," said barber shop owner James Milford. "There are several little pieces, some parts of machinery ... there's been a lot of pieces about 3 feet wide."

Two hours after the shuttle had been expected to land, the giant screen at the front of Mission Control showed a map of the Southwest United States and what should have been Columbia's flight path.

"A contingency for the space shuttle has been declared," Mission Control somberly repeated over and over.

NASA Administrator Sean O'Keefe was meeting with the astronauts' families, who had been waiting at Kennedy Space Center for the shuttle's return, spokeswoman Melissa Motichek said. Six of the seven astronauts were married, and five of them had children.

It was the 113th flight in the shuttle program's 22 years and the 28th flight for Columbia, NASA oldest shuttle.

In 42 years of U.S. human space flight, there had never been an accident during the descent to Earth or landing. On Jan. 28, 1986, space shuttle Challenger exploded shortly after liftoff.

The shuttle is essentially a glider during the hour-long decent from orbit toward the landing strip. It is covered by about 20,000 thermal tiles to protect against temperatures as high as 3,000 degrees.

On Jan. 16, shortly after Columbia lifted off, a piece of insulating foam on its external fuel tank came off and was believed to have hit the left wing of the shuttle. Leroy Cain, the lead flight director in Mission Control, assured reporters Friday that engineers had concluded that any damage to the wing was considered minor and posed no safety hazard.

The shuttle was at an altitude of about 203,000 feet over north-central Texas at 9 a.m., traveling at 12,500 mph, when Mission Control lost all contact and tracking data.

Gary Hunziker in Plano, Texas, said he saw the shuttle flying overhead. "I could see two bright objects flying off each side of it," he told The Associated Press. "I just assumed they were chase jets."

"The barn started shaking and we ran out and started looking around," said Benjamin Laster of Kemp, Texas. "I saw a puff of vapor and smoke and saw big chunk of material fall."

Former astronaut John Glenn and his wife were watching on television at their home in Maryland.

"Anytime you lose contact like that, there's some big problem. Of course, once you went for several minutes without any contact, you knew something was terribly wrong," Glenn said.

The Columbia crew was relatively inexperienced. Only three of the seven had flown in space before: the shuttle's commander, Rick Husband, Michael Anderson, and Kalpana Chawla. The other four were rookies: pilot William McCool, David Brown, Laurel Clark and Ilan Ramon.

Security had been extraordinarily tight for their 16-day scientific research mission because of the presence of Ramon, the first Israeli astronaut.

Ramon, 48, a colonel in Israel's air force and former fighter pilot, had survived two wars. He became the first man from his country to fly in space, and his presence resulted in an increase in security, not only for Columbia's launch, but also for its planned landing. Space agency officials feared his presence might make the shuttle more of a terrorist target.

"The government of Israel and the people of Israel are praying together with the entire world for the safety of the astronauts on the shuttle Columbia," Prime Minister Ariel Sharon (news - web sites)'s office said in a statement.

Dr. Yael Barr of the Israeli Aerospace Medicine Institute was waiting at the landing strip for the astronauts' return.

"When the countdown clock, when it got to zero and then started going, instead of counting down, counting up and they were still not there, I told my friend, 'I have a bad feeling. I think they are gone.' And I was in tears," Barr said.

Columbia's crew had completed 80-plus scientific research experiments during their time in orbit.

Just in the past week, NASA observed the anniversary of its only two other space tragedies, the Challenger explosion, which killed all seven astronauts on board, and the Apollo spacecraft fire that killed three on Jan. 27, 1967.

On the Net: http://spaceflight.nasa.gov

The US space agency has begun an internal and external, independent investigation into the loss of the orbiter Columbia. These are the facts as they are understood.

  It is a perfect day to land the shuttle. There are no weather issues to complicate the orbiter's final approach to the Kennedy Space Center in Florida. The touchdown is set for 0916 EST (1416 GMT).

• At 0853, the first indications of problems begin to emerge. There is a loss of temperature sensors in hydraulics systems in the trailing edge of the left wing.
  
  
• At 0856, sensors in the left main gear tyre-wheel well report a temperature increase.
  
  
• At 0858, three temperature sensors on the left side of the vehicle stop working. At this stage, Columbia is at an altitude of nearly 40 miles; its speed is in excess of Mach 18 (18 times the speed of sound - 13,200 mph). It is still 1,400 miles from the Kennedy runway. Columbia is in a left-bank, with its wings angled about 57 degrees to the horizontal.
  
  
• At 0859, sensors monitoring tyre temperatures and pressures report no data. Mission control contacts the shuttle: "Columbia, Houston. We see your tyre-pressure messages. We did not copy your last."
  
  
• There is a very short delay before the shuttle comes back: "Roger... erm..." The line is lost.
  

  The track Colombia should have taken over the US

• For the next few minutes, repeated calls are made to Columbia - but with no response.
  
  
• Witnesses in Texas describe hearing a huge bang and see several white trails in the blue sky.
  
  
• Just after the scheduled landing time, Nasa puts into practice its contingency action plan. President Bush is telephoned at Camp David to inform him that the shuttle is lost.
  
  
• Rescue units are scrambled to search in Texas for the shuttle and its seven astronauts. Local emergency services are asked for assistance.
  
  
• At 0930, a Mishap Investigation Team is assembled to probe what went wrong. The order is sent to preserve all computer data relating to the last few minutes of the flight. The order is also sent to secure all pre-flight inspection and servicing records for Columbia.
  
  
• Efforts also begin to set up an external Mishap Investigation Team. It will contain personnel from the US Airforce, Navy and other agencies.
  
  
• Nasa Administrator Sean O'Keefe speaks with the families of the astronauts lost on Columbia.
  
  
• President Bush also calls the families of the astronauts lost on Columbia to offer his sympathy and to promise a full investigation.
  
  
• The Texas public safety department reports more than 2,000 debris fields, scattered from the small town of Nacogdoches, about 170 miles south-east of Dallas, to the Louisiana border.

Nasa said the shuttle was about 200,000 feet up and travelling at 12,500 mph (20,000 km/h) at the time.

Click here for pictures of the astronauts' final mission

Initially, the shuttle's re-entry over Texas appeared to be going to plan

However, the plume following the shuttle soon thickened, indicating there was trouble

As pieces began to fall from Columbia, explosions of light could be seen from the ground

Debris from Columbia soon began landing over a wide area of Texas, including just outside Rice High School in Texas

People pay their respects at the site of fallen debris found in Nacogdoches, Texas