U.S. investigators say a badly burned piece of thermal tile from the doomed space shuttle Columbia might hold clues to why the spacecraft disintegrated upon re-entry from orbit. The location where it was found and its original place on the orbiter make it a potentially important discovery.

The search for shuttle debris has turned up a fragment of a thermal tile further west in the state of Texas than any other piece of Columbia so far. This means it was probably among the earliest pieces to break away from the shuttle and might offer a glimpse into how the calamity that led to the break-up began.

The independent board investigating the disaster says the fragment's shape and thickness indicate that it came from the top surface of the troubled left wing near where it attached to the shuttle's body.

This could be significant because the board suspects something penetrated Columbia's left wing, allowing extremely hot gases to enter it. Readings from wing sensors showed a dramatic temperature rise minutes before the break-up.

Panel chairman Harold Gehman says tile experts will screen it and other recovered tiles. But he notes that investigators are a long way from explaining what caused the tragedy. "The tiles that we have are pretty beat up. It is not until we get enough of them to put them together in some kind of order that we are able to develop a picture of what happened," he explained. "They are beginning to talk to us now. Some things are beginning to emerge, but no answers are beginning to emerge."

Mr. Gehman says searchers have recovered only 10 percent of the shuttle from Texas and neighboring Louisiana, where it fell in thousands of pieces after racing toward a landing at 18 times the speed of sound. Only a tiny fraction of the left wing is found.

The investigators emphasize that they are at the beginning of their inquiry, still learning about the spacecraft and the infrastructure that supported it. Mr. Gehman's team is spreading around the United States this week to interview workers and managers at several regional offices of the U.S. space agency NASA and its contractors.

Unlike the probe of the 1986 shuttle Challenger explosion, no clear accident cause has emerged nearly one month after the Columbia disaster. As a result, Mr. Gehman says the investigation is looking into every aspect of shuttle structure, operations, maintenance and NASA management policies. "Since we don't know where we are going, I can't tell you where we are. It's hard for us to say how this is going to come out at the end," he said. "I just wouldn't want to get into any speculation on that, except to say that in order that we can ensure that NASA gets started on fixing things, when we have a preliminary finding, we'll say that."

On Thursday, the U.S. House of Representatives committee that oversees NASA will hold separate hearings on how the Columbia accident is affecting the space agency's programs.