By Dr David Whitehouse BBC News Online science editor

It was sent to the Barksdale Air Force Base collection point

The Columbia Accident Investigation Board has just released this photograph showing a heavily damaged, almost melted piece of the left wing.

Engineers say the seared and scarred fragment is from the area immediately adjacent to the left wheel well, where many think the orbiter's problems during re-entry started.

Experts say the damage is significant and unusual.

The heat-resistant tiles on the outer surface of the wing appear almost lava-like in appearance, indicating exposure to extreme heating.

The left-wing wheel well is of intense interest because telemetry from that part of the shuttle showed abnormal temperature increases and sensor failures just minutes before Columbia broke apart on 1 February.

Some engineers believe a breach in the left wing allowed hot gas to penetrate in or around the wheel well as the shuttle encountered maximum atmospheric heating on its descent. The breach presumably worsened until a structural failure occurred.

Experts say that although the tile damage indicates exposure to extreme temperatures, it is interesting to note the underside of the debris shows no signs of such intense heating.