Nasa has identified the remains of Israeli astronaut Ilan Ramon, as teams from the US space agency travel to California and Arizona to investigate reports of debris from Columbia landing there.

If confirmed, the reports would suggest that the space shuttle began to break up earlier than previously thought.

Ramon took an Israeli air force flag into space

On Tuesday, President George W Bush paid tribute to each of the seven astronauts killed in Saturday's disaster and vowed that the "cause of discovery" would go on.

Colonel Ramon was identified via DNA and dental records, according to Israel Radio.

His remains will be returned to Israel for a military burial following a funeral in Texas, where they were located.

Nasa has reportedly recovered some remains of at least three other crew members but has not yet released their names.

Disintegration

The Columbia was dramatically filmed disintegrating in the skies over Texas where many of the 3,500 debris sites are located.

Our nation shares in your sorrow and in your pride President Bush

Click here for excerpts of George Bush's speech

Nasa now believes the disaster may have begun earlier than previously thought.

"Early debris early in the flight path would be critical because that material would obviously be near the start of the events," said a senior Nasa official, Michael Kostelnik.

"It would clearly be very important to see the material earliest in the sequence."

General Kostelnik added that the space agency still did not really know the cause of the disaster and it needed to acquire the maximum possible data.

Meanwhile, an unmanned Russian cargo space ship has successfully docked at the International Space Station to deliver fresh supplies and fuel to the three astronauts who remain on board after the suspension of the shuttle link.

'Flying forever'

Speaking at the Johnson Space Center in Houston, President Bush offered "the respect and gratitude" of the people of the United States to the five men and two women who perished.

SHUTTLE CREW

Commander Rick Husband, US Pilot William McCool, US Michael Anderson, US David Brown, US Kalpana Chawla, US Laurel Clark, US Ilan Ramon, Israel

Click here for profiles

"Today we remember not only one moment of the tragedy but seven lives of great purpose and achievement," he said in front of the 10,000 people who had gathered for the service.

The space programme must nevertheless continue, he insisted, echoing former President Ronald Reagan's speech after the Challenger space shuttle was lost in 1986.

Barry McCool, father of Columbia's pilot William McCool, was also at the Houston service to remember his son.

"We're all very proud of Willy. He was a magnificent son and a great brother. We really can't say any more than that. He always wanted to fly and now he's flying forever."

Mr Bush has sought a 3.1% increase in funding for the Nasa space agency for next year, to $15.5bn, including $3.9bn for the space shuttle programme itself.

The White House has played down reports that under-funding may have played any role in the loss of Columbia.

Painstaking search

A college professor in California has told Nasa that he saw tiles dropping from the shuttle as he watched its descent towards Florida through a telescope.

SHUTTLE BREAK-UP

Re-entered atmosphere at 12,500 mph (20,000 km/h) Disintegrated 40 miles (65 kilometres) above the Earth Debris scattered over Texas and Louisiana - reports now being checked of sightings in California and Arizona

Click here for a step-by-step guide to the disaster

Officials plan to assemble the parts and reconstruct the shuttle to try to establish what happened in its final moments, but the task is monumental.

"Debris may not be located for weeks, for months, for years. Some of the debris may never be found," said Sheriff Thomas Kerss of Nacogdoches County, Texas, where much of the wreckage has been found.

An arrest warrant may be issued on Wednesday for at least one person accused of removing debris, he said.

It is an offence to take any wreckage of the shuttle because it is government property. It may also be dangerous to handle debris, Nasa has said.

Shuttle programme manager Ron Dittemore has confirmed that Nasa engineers are still focusing on a piece of insulating foam as the leading suspect in causing the problem which destroyed the Columbia.

The agency is considering the possibility that the impact from the foam - which fell off and struck the shuttle moving as fast as 2,400 km/h (1,500 mph) during lift-off - damaged the shuttle's heat-resistant tiles near a wheel well, one of the most vulnerable parts of the vehicle.

US shuttles have been grounded indefinitely since the disaster struck and the future of the ISS is in question given Russia's limited capacity for supplying the space station with its spacecraft alone.