Search efforts fail to find shuttle's crucial pieces

He said those items would include parts of the left wing, data recorders and certain pieces of insulation and tiles.

The widening search now extends from Louisiana to California.

In Texas alone, officials have identified 38 counties with debris, while pieces have turned up in two dozen Louisiana parishes. And NASA investigators are checking California and Arizona for debris as well.

The shuttle was composed of about 2 million parts, many of which shattered into pieces as small as a nickel.

Bill Waldock of Embry-Riddle Aeronautical University in Arizona said any of the craft's 20,000 insulating tiles or metal components from the left wing would be significant.

At least two possible wing sections have been discovered in east Texas, although authorities did not know from which side of the shuttle they came. A robotic underwater camera was brought in Wednesday to help search a reservoir along the Texas-Louisiana border where there were reports of debris falling.

A patch of foam insulation that broke off from the shuttle's external fuel tank during launch and struck tiles on the underside of the left wing had been the focus of the probe into the possible causes of Columbia's destruction. After days of analysis, NASA backed away Wednesday from the theory that the foam might have been the cause of the accident.

Instead, Dittemore said investigators are focusing more closely on the frantic effort of Columbia's automatic control system to hold the speed of the spacecraft stable despite increasing wind resistance, or drag, on the left wing.

The insulating tiles protect the underbelly and the wings of the shuttle from searing heat. Each is stenciled with a code to tell engineers where it was located on the craft. Tiles that peeled off the left wing had been considered crucial to the probe.

Waldock said some pieces, such as the nose cone, could help investigators rule out other potential causes of the disaster. "It didn't look like the nose cone had much thermal damage at all; it's not even really scorched," he said.

Another expert said the pattern of where pieces fell would also offer important clues. The heat of re-entry would have peeled back the shuttle, layer by layer, heating and breaking off pieces in succession as it streaked eastward through the atmosphere, noted William Ailor, director of the Center for Orbital and Re-entry Debris Studies at The Aerospace Corp. in El Segundo, Calif.<