Retired US Navy Admiral Harold W Gehman, the man appointed to lead the independent inquiry into the space shuttle Columbia disaster, has arrived in Houston.

He and his team of investigators spent the day at Nasa's Johnson Space Center being briefed on how the investigation is going so far.

"We continue to investigate the foam as a possible root cause."
- Ron Dittemore, space shuttle programme manager

Admiral Gehman, who co-chaired the independent commission that investigated the attack on the USS Cole in Aden, Yemen, in 2000, has already visited the area of eastern Texas where much of the shuttle debris landed.

The search for wreckage, which Nasa experts hope will yield vital clues as to why the shuttle broke up on re-entry into Earth's atmosphere, continued throughout Thursday, although heavy rain and sleet made the task more difficult.

'Looters' arrested

The recovery effort has already been hampered by members of the public trying to keep parts of the shuttle, reportedly as souvenirs.

Admiral Gehman's team will hold their first meeting on Monday

Two Texans were charged on Thursday with stealing pieces of debris - they could each face up to 10 years in prison and a $250,000 fine. 

But police reported that an amnesty offered for anyone handing over looted shuttle parts had resulted in a number of pieces being turned in.

Members of the public have until 1800 EST (2300 GMT) on Friday to hand over debris without facing a penalty, US attorneys said. 

Foam theory

Nasa is particularly interested in finding parts of the shuttle's left wing which had recorded a rise in temperature and increase in drag just minutes prior to the disaster.

The agency has moved away from the theory that insulation foam seen to peel away from Columbia's external tank on launch and hit the vehicle's left wing was responsible for the accident.

But at the latest technical press briefing Ron Dittemore, Nasa's space shuttle programme manager, said that this did not mean the possibility had been discounted.

"They were soldiers and scientists and doctors and pilots, but above all they were explorers"
- Vice-President Cheney

He said that a number of tests would be carried out, checking the damage that such an impact could have incurred.

"We continue to investigate the foam as a possible root cause," Mr Dittemore said.

"Although I said before that we thought it unlikely that this was the cause it is not being discounted. No possibility is being discounted," he added.

Top Secret Device

Reuters news agency reported that around the town of Bronson, near the Texas-Louisiana border, hundreds of national guardsmen joined federal agents and volunteers to search for what was believed to be a top-secret device from the shuttle.

People involved in the search said they were given written instructions on what the device looked like as well as a picture of a plate attached to it which read "Secret Government Property".

SHUTTLE BREAK-UP

A report in a Texas newspaper on Thursday said searchers were looking for a communications device that relayed encrypted messages between the shuttle and the ground.

More than 12,000 remnants, many as small as coins, have so far been recovered.

Much of the debris landed in heavily wooded areas and pastures, making the search difficult.

Mr Kerss said driving rain meant that it was difficult for heavy vehicles and equipment to cross the terrain to collect the debris.

The cold conditions meant the horses being used in the search were becoming more easily tired and hundreds of people scouring the countryside had to be sent out in shorter shifts.

Nonetheless several significant pieces of wreckage were found near the town of Hemphill, including parts of the fuselage, nose cone and computer circuits, said Marq Webb, a spokesman for the US Forest Service.

Meanwhile on Thursday US Vice-President Dick Cheney presided over a service at the country's National Cathedral in Washington, where a stained glass window holds a piece of Moon rock, to honour the crew of Columbia.

"They were soldiers and scientists and doctors and pilots, but above all they were explorers," Mr Cheney said.

Later on Friday a memorial service will be held on the runway where the ill-fated Columbia was supposed to land at the Kennedy Space Center.

 
"It's very difficult, as if I'm with them and I try to imagine what they went through," Eliezer Wolferman, father of Israel astronaut Ilan Ramon said.

"One second is like 20 years."

News of the astronauts' probable final moments comes amid reports that the US space agency (Nasa) was warned at least nine years ago that a space shuttle flight could end in disaster if tiles protecting critical wing parts were damaged by debris.

Footage examined

Nasa, which has expanded its search for debris, said it hopes to review footage taken by a US military helicopter above Texas. This may include pictures of the shuttle's re-entry into the Earth's atmosphere.

Nasa officials also want to examine amateur video footage taken of the shuttle over California which is said to show the craft breaking up.

Ilan Ramon's brother, Gadi, said he had been told by Nasa officials that the astronauts most likely perished as soon as the shuttle began to disintegrate.

"We apparently will never know if they suffered," he told an Israeli newspaper.

Prior warning?

As investigations continue, damage to the heat-resistant tiles is emerging as a leading theory for Columbia's catastrophic end.
Paul Fischbeck, engineering professor at Carnegie Mellon University, said he had warned about the tiles in a 1994 study for Nasa. 

The most vulnerable spots on the shuttle were the undersides of both wings close to the fuselage and right under the crew compartment, he said. 

"There are very important tiles under there; if you lose the tiles on those stretches... it can cause the shuttle to be lost," Professor Fischbeck told the Associated Press news agency.

Nasa shuttle programme manager Ron Dittemore has confirmed that Nasa engineers are still focusing on the possibility that a piece of insulating foam - which fell off and struck the shuttle during lift-off - damaged the shuttle's heat-resistant tiles near a wheel well. 

However, Mr Fischbeck stressed that it was still not certain if debris had been the cause of the disaster.

Several Nasa employees and former astronauts who worked on previous missions have said that almost all shuttles return to Earth with tiles damaged by space debris.

SHUTTLE CREW

Nasa teams are now also in California and Arizona to investigate reports of debris landing there. 

Authorities said that material found in the two western states could be from the wings. This could lend weight to the possibility that the shuttle was experiencing problems sooner than originally 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."

Remains recovered

Ilan Ramon's remains have now been identified using DNA and dental records, Israeli reports say.

His body will be flown to Israel in the coming days after a service in Texas.

"This is a relief to all of us, especially the family," Brigadier General Rani Falk, an Israeli air force attache in Washington, told the Associated Press.

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

During a memorial service for Ramon and his six colleagues on Tuesday, President George W Bush led the tributes and vowed that the "cause of discovery" would go on.