-- After orientation trips to various NASA centers last week, the panel investigating the space shuttle Columbia disaster settled in to new offices here today and began a new stage of its investigation that will be largely guided by a huge chart called a "fault tree."

Used for decades to analyze tragedies ranging from car crashes to the Three Mile Island nuclear accident, it is a tool that helps lay out the full range of possible causes.

Fault trees begin with an outcome. The top of the one for Columbia says simply: "Loss of Vehicle."

It then might branch down to list the major system failures that could have contributed to that -- problems in the engine, flight control computer system, structure and so on. Beneath those may be possible reasons for, say, engine failure such as a glitch in the fuel line or a failed electronics box. Then the tree might list what might have caused a glitch in the fuel line, such as a crack or an obstruction. Each subsequent layer is more specific.

As certain scenarios are eliminated, the tree is "pruned." If the tree is constructed correctly, the last remaining branch should explain what happened.

"Fault trees are important because they give a more structured, objective approach to ensuring you leave no stone unturned," said W. Brian Keegan, who served as NASA's chief engineer until April.

With 6,800 single failure points on the fault tree that can lead to the loss of the craft or crew, the challenge for the engineers working on it will be to make sure every possible scenario, no matter how unlikely, becomes part of it.

Shortly after Columbia's fate was confirmed Feb. 1, NASA officials pulled out a more than 200-page handbook that documents fault trees for space shuttle incidents and updated some of the branches, specifically those that deal with the layer of heat-resistant tiles around the spacecraft and the wings, which were not as detailed as might be needed. Over the past two weeks, NASA has removed all but a few of the branches, among them those that said that there was a navigation glitch and that the landing gear deployed early.

Adm. Harold W. Gehman Jr., the head of the commission, said last week that the group will work off the NASA fault tree and although it will not "double check everything," it will independently verify why certain paths were ruled out and may reopen them if it deems necessary.

NASA engineers for the most part have focused their analysis on the possibility that Columbia was torn apart when the layer of heat-resistant tiles on the left wing was pierced. That might have allowed searing gas to tear through the shuttle from the inside. But what remains a mystery is what caused that hole.

Gehman's 11-member panel has been traveling to NASA facilities that were involved in constructing, upgrading or maintaining parts of the space shuttle. On Saturday, they were in Louisiana, at a Lockheed Martin Corp. facility where the ship's external fuel tanks are built and sprayed with the foam insulation that fell off and hit Columbia's left wing 81 seconds after liftoff. Before that the board was at NASA's Marshall Space Flight Center in Huntsville, Ala., where the engines were designed, and the Kennedy Space Center in Florida, where the shuttle's tiles and other systems were last repaired.

Panel spokesman Steve Nesbitt said the investigators will spend the next few days poring over the documents gathered from their trips and will be listing people to be interviewed and citing reports needed for the investigation.

Adm. Harold W. Gehman Jr. and his investigative panel have been visiting NASA centers.