A small but key piece of wreckage from the space shuttle Columbia's left landing-gear door, shown in photos released over the weekend, may help investigators zero in on the spot where superheated air first breached the vehicle's skin, according to a source close to the investigation.

The piece -- a pin with a u-shape at one end, which is part of the gear door latch -- will likely provide a clue to what was happening to the doomed vehicle and the direction of the lethal flow of hot gases, the official said.

This and other evidence lend growing credence to the theory that an initial breach in Columbia's protective heat shield occurred on or near the leading edge of the left wing, flowed into the wing with the effect of a blowtorch and into the wheel well, where it built up heat and pressure until it eventually blew open the door. "That's a theory that has a lot of support," the source said. "Can we prove it yet? No."

The investigation, while considering other possibilities, has focused heavily on a scenario in which the damage that triggered the accident resulted from the impact of foam or other debris that appeared to break free of the shuttle's external fuel tank 82 seconds after the Jan. 16 liftoff. In film and video viewed after the launch, the debris seemed to strike the shuttle's underbody on or near the left wing, possibly in more than one place.

Various types of tests are underway to refine the investigators' knowledge of what was in that debris and where it struck, but the official said at least one approach -- the effort to conduct spectral analysis of the video -- is proving fruitless.

During the accident, the recently identified door pin became partially coated with an unknown metallic substance that may be aluminum -- the main component of the shuttle's structure -- but has yet to be analyzed, the official said.

One of the prongs, or flanges, of the pin's u-shape is melted. "The thing is made out of titanium, which has an extremely high melting point, so for that part of the flange to be eaten away, you had to have enormous heating," the official said. "But the other flange is fine, not eaten away. The suspicion is that whatever latch was in between those two flanges was masking the heat" for the undamaged part.

This indicates the damage was caused by the initial heating event, not by the mayhem of the shuttle's ultimate breakup. In that case, the damage would have been general rather than targeted on the one flange.

In this scenario, the landing-gear door would have remained in place until very late in the sequence of events that led to the destruction of the shuttle as it reentered the atmosphere Feb. 1, the official said. Other small pieces of debris from the landing-gear door also indicate that the door was in place as pressure built, puckering the edges of the aluminum skin so that the hot gas began to open vent holes around the edges.

Investigators have been studying the properties of the heavy-duty carbon fiber panels that protect the leading edge of the wing, which are bolted on. Among other things, engineers are looking into the possibility that these elements had been weakened or made more vulnerable to damage as a result of defects or wear and tear, signs of which have been found on similar elements from other shuttles in the fleet.

Wind tunnel tests have shown that, to produce the kind of excess drag observed during Columbia's descent, several of these protective panels would have had to be missing.

Analysts at Wright-Patterson Air Force Base in Dayton, Ohio, are studying radar images of an object seen moving away from Columbia one day after it reached orbit. By comparing that signature to the radar signals of shuttle protective tiles, carbon panels and other hardware, they hope to determine whether the mystery object was a piece of the shuttle whose loss helped trigger the fatal chain of events.