Working through a process of elimination, NASA engineers are focusing on 10 major failure scenarios - and combinations thereof - to explain what went wrong during the shuttle Columbia's catastrophic re-entry Feb. 1. Several of those scenarios, outlined in general in a March 7 report, assume a breach at or near the leading edge area of Columbia's left wing and a resulting plume of super heated air that burned its way into the wing's interior. Heat damage in recovered wreckage and telemetry from the doomed ship indicate the deadly plasma may have worked its way into the left landing gear wheel well and then burned through seals around the left landing gear door.

Many NASA investigators believe such a scenario, or a variation of that chain of events, best explain the sensor failures and temperature readings downlinked during Columbia's final minutes as well as burn damage on wreckage recovered to date. But other failure scenarios are possible as well and NASA has assigned teams to work through each one in exhaustive detail to determine which ones best fit the existing data and debris damage patterns, agency sources say. The analysis is being carried out as part of NASA's support of the independent Columbia Accident Investigation Board, which has the final say in determining what happened to Columbia and, more important, why it happened.

While the leading edge of the left wing is clearly an area of prime focus, engineers have not yet ruled out a burn through from the bottom of the wing, either in an area of presumably widespread tile damage or because of damaged tiles and seals at or near the landing gear door itself. But nearly all of the scenarios under discussion involve presumed breaches toward the front, inboard section of the wing, from the landing gear door forward. And again, NASA sources and others close to the accident investigation board say a breach in the leading edge area is among the most plausible failure scenarios to emerge so far.

In other developments, sources told CBS News late last week that Randy Stone, a respected former flight director and now deputy director of the Johnson Space Center, will take over Linda Ham's major duties supporting the independent Columbia Accident Investigation Board.

Ham, chairman of NASA's mission management team, played a key role in the decision, during Columbia's mission, to accept an analysis indicating the shuttle could safely land despite potentially significant wing damage from the apparent impact of foam debris from the shuttle's external tank during launch. Ham also oversees NASA's Mishap Response Team.

Late last month, board Chairman Harold Gehman asked NASA Administrator Sean O'Keefe to remove senior shuttle managers who played a role in the conduct of Columbia's mission from duties supporting the ongoing accident investigation. The stated goal was to eliminate any appearance of a conflict of interest.

Gehman's letter to O'Keefe did not list any names. But sources said the board wanted senior managers like Ham, who ultimately will face the accident board as part its investigation into how NASA managed Columbia's mission, removed from direct roles in the ongoing investigation.

"It has become apparent that some of the chief managers of the investigation which NASA and this board share are also members of these boards that we're going to be looking at," Gehman said March 4. "We are then put into the place of having the investigators investigate themselves."

"That's not exactly true because NASA is not investigating management issues. Only we are investigating management issues. But ... I can't possibly have key investigatory managers also be the people whose performance we're looking at in other areas."

O'Keefe initially balked at making any immediate changes, but he later agreed to assign NASA personnel not directly involved in the conduct of shuttle missions to key teams supporting the board's investigation.