What do the tiles do?

The tiles make up the thermal shield that is designed to protect the shuttle on its journey back to Earth.

The spacecraft encounters intense temperatures as it re-enters the planet's atmosphere, nose up and "belly first".

Once tiles are lost and the heat shield is damaged, the fate of the crew is sealed Peter Bond

To withstand temperatures of up to 2,000 degrees Celsius, it is coated with 24,000 such tiles.

The tiles line the underside of the shuttle, including the leading edge of each wing, and the nose.

They are of different types but the most resistant ones are black tiles, made from reinforced carbon.

Why does evidence point to the tiles?

Nasa fears that a piece of fuel-tank insulation that fell off at launch may have damaged one or more tiles.

Temperatures on the left side of the shuttle rose significantly before it burst into flames, especially near the left wheel recess.

The heating effect would have buffeted Columbia and could have caused more tiles to fall off.

Could the damage have been repaired?

This has been a matter of speculation. Some are asking whether more could have been done before landing.

The space community is in mourning

Peter Bond, space science adviser to the UK Royal Astronomical Society, says Nasa looked into whether they could be repaired as early as the 1970s.

The space agency found there was no safe way to do it, he says, especially for repairs to the underside of the shuttle.

"Each tile is unique - there are hundreds of tiles and they are all a different shape and size," he told BBC News Online.

"Once tiles are lost and the heat shield is damaged, the fate of the crew is sealed."

Nasa insists there was nothing on board that the crew could have used to repair missing or broken tiles.

In any case, it believed at the time that the tile damage was no reason for concern.

Have tiles caused problems before?

There is a long history of problems with thermal tiles during the space shuttle programme, including with Columbia itself.

In the early days of research, the glue that held the tiles to the bottom of the shuttle would not stick. The hitch was later solved by engineers.

Columbia, which was the first shuttle to fly in space, got off the ground in 1981. Its launch was delayed after more than 150 heat tiles were damaged during a test flight in 1979.

Another space shuttle, Atlantis, was reported to have lost as many as 175 protective tiles, in a classified mission in 1988.

The US space shuttle fleet has been re-fitted many times over the years, including a complete overhaul of the thermal protection system.

What about other causes of the disaster?

Nasa officials say they cannot yet confirm the cause of the crash but they say there was a significant rise in temperature on the left side of the shuttle shortly before it disintegrated.

Independent experts say it is too early to rule out other factors but a thermal rather than structural problem seems likely.