The space shuttle Columbia may have been shedding heat-protection tiles as it flew over the United States on its re-entry into the Earth's atmosphere, Nasa officials have said.

She was proud to be representing her country Astronaut Laurel Clark's aunt

Families remember loved ones

At the same time, officials confirmed that remains of some of the seven astronauts on board Columbia had been found.

Nasa has vowed to leave "no stone unturned" in its investigation into why the space shuttle disintegrated just minutes before its scheduled landing on Saturday.

Heat sensor readings have shown that the temperature of the left side of the craft started to rise as it passed over California and New Mexico and that the shuttle was experiencing increased drag on the same side.

	:	Shuttle disaster
How Columbia broke up over Texas

Space Shuttle Programme Manager Ron Dittemore said the increased drag could have been an indication that the shuttle had lost heat-protection tiles or that the tiles had become uneven.

"But I have to caution you that this is a very fluid situation and what we understand today may change in the coming days," he stressed.

The spacecraft's ceramic tiles are crucial in shielding the shuttle from temperatures in excess of 1,000 Celsius re-entering the Earth's atmosphere.

Experts are carrying out a painstaking analysis of the mass of data transmitted back from the shuttle in the final minutes of its flight.

Another key part of the investigation will be analysing the pieces of the shuttle which rained down over the southern US - a process likely to take months.

Possible damage to the tiles on Columbia's left wing had already been flagged up as a cause for concern.

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The wing was hit by a piece of insulating foam which peeled away from the external fuel tank a little more than a minute into Columbia's launch on 16 January.

The incident was spotted and checked at the time, Nasa officials said. But they acknowledged they could not now rule out a connection.

First flight: 1981 Orbiting speed: 17,500 mph Landing weight: 105 tonnes Crew (for this mission):7

Columbia: Space pioneer

As Americans mourned the deaths of the shuttle's seven astronauts, police teams scoured large areas of Texas and Louisiana for shuttle fragments.

Already, accusations are being levelled that Nasa chiefs ignored a series of safety warnings.

But these were rejected by Nasa administrator Sean O'Keefe, who said every safety concern was tackled before a shuttle launch.

The disaster has raised questions about the future of the 16-nation International Space Station (ISS).

However, Russia on Sunday went ahead with the launch of an unmanned cargo vessel to serve the ISS.

Families' pride

Shock and grief at the disaster has been expressed around the world.

India, where one of the crew was born, and Israel, which had hoped to celebrate the return of the first Israeli astronaut, joined the US in mourning.

An astronaut's helmet lies in a field in Texas

President George W Bush led the US in prayers on Sunday as the astronauts were remembered at church services nationwide.

The crew's relatives have been speaking of their grief, but also of their pride.

The family of astronaut Laurel Clark said she had sent them e-mails on Friday speaking of her love of space.

"She was proud to be representing her country and dealing with advanced scientific projects from all over the world," her aunt Betty Havilan told local television in Wisconsin.

Debris danger

The shuttle broke up 40 miles (65 kilometres) above the Earth, scattering debris and human remains over a huge area.

FBI agents joined local police officers in the search.

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

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A team of divers was called in to search a reservoir on the Texas-Louisiana border, where a large piece of shuttle debris was seen hitting the water.

About three dozen people needed hospital treatment for burns and breathing problems after handling pieces of the wreckage.

Nasa warned residents to avoid any debris.

Columbia, had been due to land at 0916 EST (1416 GMT) on Saturday at the end of a 16-day mission. Contact was lost at about 0900 EST.

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

In 42 years of human spaceflight, Nasa has never before lost a space crew during landing.

In 1986, the space shuttle Challenger exploded shortly after lift-off, with the loss of all seven crew on board.