The shuttle Columbia may have been shedding heat-resistant tiles as it flew over the US on its catastrophic re-entry into the Earth's atmosphere.

The Last Moments

What happened and when At 0858... the vehicle is also experiencing increased drag on its left side

Read more

At a technical briefing on Sunday, the US space agency, Nasa, gave more details about anomalous sensor readings recorded during the last few minutes of the shuttle's flight.

These show unexpected heating was occurring on the left-hand side of the vehicle as it passed over California and Nevada.

The sensor data also reveal Columbia was experiencing increased drag on the left side, and the shuttle's flaps were moving to compensate for it.

Changing circumstances

Space Shuttle Programme Manager Ron Dittemore said the detection of increased drag could have been an indication that the vehicle had lost some heat-resistant tiles or that the tiles had become uneven.

"But I have to caution you, we're in very early stages of our analysis and we're looking over a lot of data," he told reporters. "This is going to be fluid and it is going to change and we could contradict ourselves from day to day."

	:	Shuttle disaster
How Columbia broke up over Texas

Investigators have taken a statement from an eyewitness in California who claimed to have seen debris coming away from Columbia as it passed over the state.

Speculation exists that because temperature readings rose in the left-hand wheel well above what was normal, a landing-gear door may have come off.

Mr Dittemore would not be drawn into discussing this scenario, however.

"We certainly know that the wheel well is one of our sensitive areas thermally," he said. "I know what you're thinking and I'm thinking the same thing, but I can't go beyond what I have said because I don't want to jump to conclusions."

First meeting

The Nasa official stated there were 32 seconds of data after the signal was lost to Columbia that could possibly still be retrieved and analysed for clues.

Mr Dittemore reiterated that investigators had no information to indicate that insulation foam seen to fall off the shuttle's external fuel tank on launch had caused any damage to the orbiter.

Nasa Administrator Sean O'Keefe has announced the members of the Space Shuttle Mishap Interagency Investigation Board, which will conduct the independent review into Columbia's loss.

It will be headed by retired US Navy Admiral Harold W Gehman Jr, who co-chaired the independent commission that investigated the al-Qaeda attack on the USS Cole in Aden, Yemen, in 2000.

The Gehman board will have its first meeting on Monday.

Nasa's own internal investigation is currently concentrated on retrieving as much debris from the shattered shuttle as possible.