Space Center, Houston ? The Columbia investigation board is trying to zero in on what caused the hole in the shuttle’s aluminum skin that apparently allowed superhot gases to penetrate the wing and destroy the spacecraft.

The investigators have said a gash or some other kind of breach probably doomed the spacecraft.

Columbia broke up in the sky over Texas, killing its crew of seven and leaving a trail of debris from Texas to Louisiana.

No debris has been found west of Texas, although New Mexico Gov. Bill Richardson said Friday NASA believed a piece of space shuttle wing might have fallen in the Sandia Mountains there. A search was planned today.

Meanwhile, the board is analyzing possible causes for the breach. One possibility, raised from the very start of the investigation, is that the shuttle was damaged by a hard piece of foam insulation that fell off the external fuel tank during liftoff.

On Friday, flight director Leroy Cain said the foam debris immediately came to mind Feb. 1 during the shuttle’s final moments when he was first told of sensors breaking down in the spacecraft’s left wing as it descended.

“That gave me pause. That was the first time I had thought about that debris hit we took on ascent,” Cain said. But he said when he learned about the sensor readings, he did not imagine it would spell the end for the shuttle and its crew: “I did not think that we would lose it.”

Seven minutes after the first indication of a problem, Columbia broke apart.