The remains of the six US crew members have been taken to a Defence Department mortuary in the state of Delaware. Israeli astronaut Ilan Ramon was buried in his country on Tuesday.

At some point the wheel could fail and send debris everywhere Internal Nasa e-mail, 30 January

Nasa said "significant amounts" of remains were found in Texas on Wednesday.

On the same day, the agency released an e-mail written before the crash that warned of potentially "catastrophic" results if there were problems with the shuttle's wheel well.

Damage to the wheel well - one of the most vulnerable parts of the shuttle - is considered a possible cause for the disaster.

'Worst-case scenario'

Warning that he was "erring way on the side of absolute worst-case scenarios", engineer Robert Daugherty's e-mail listed problems that could occur if landing gear failed to deploy properly.

We will find the problem that caused the loss of Columbia and its crew, we will fix it and we will return to flight operations that are as safe as humanly possible in pursuit of knowledge Sean O'Keefe Nasa administrator

"At some point the wheel could fail and send debris everywhere," the engineer theorised on 30 January, two days before the loss of the shuttle.

A Nasa spokeswoman said the astronauts' remains would be released to the families for private memorial services.

"This is a private matter for the families and we are trying to respect that," spokeswoman Eileen Hawley said.

On Wednesday, Nasa's top official Sean O'Keefe told the first congressional hearing into the tragedy that throughout Columbia's 16-day mission there were "no indications" the disaster would occur.

Cause unknown

It is still unclear what caused Columbia to disintegrate 40 miles (65 kilometres) above the Earth.

The first pieces of debris have started to arrive at the Kennedy Space Center in Florida, where investigators hope to reconstruct the shuttle to look for clues for what caused it to break up.

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

Mr O'Keefe was testifying before a joint House and Senate committee set up to examine the Columbia disaster and to consider the future of the national space programme.

He said the cause of the disintegration would be found and addressed.

But he called for continued support for the work of Nasa, including manned space flight as well as the use of machines.

"We know the lesson from this terrible accident is not to turn our backs on exploration simply because it is too hard or risky," he said.

"We will find the problem that caused the loss of Columbia and its crew, we will fix it and we will return to flight operations that are as safe as humanly possible in pursuit of knowledge."