The investigation into what caused the break-up of the U.S. space shuttle Columbia supports the theory that a left wing panel broke off during its mission, possibly playing a role in its demise. But corrosion along the wing's front edge might also have played a part.

Investigators have narrowed the location where devastating superheated atmospheric gases entered the shuttle during its re-entry to somewhere near the leading edge of the left wing. That is where sensors from a recently recovered data recorder show that temperatures heated up first, right after Columbia re-entered the atmosphere from its 16-day flight.

The investigators do not know where the breach in the wing was, but they say their probe points to several theories.

One option is that one of protective panels beneath the left wing detached.

One of the investigators, Air Force Major General John Barry, says the object that flew off the shuttle on its second day of flight almost certainly was one of these panels, called carrier panels. He cites analysis of data from military radars that observed the debris floating in space and tests of different types of materials that could have fallen away from the shuttle in orbit.

"We've got a lot of radar cross-section feedback, and there have been 29 various materials examined at Wright Patterson Air Force Base," said Gen. Barry. "We've concluded that, right now, only the carrier panel remains a viable candidate for the day-two object."

The investigators are also considering whether tiny holes in the wing's edge could have contributed to Columbia's demise.

They have learned that it is common for the front edges of shuttle wings to have pinholes from corrosion. General Barry says the cause of the corrosion could be zinc oxide leaching down from aging paint on the shuttle launch tower. Another culprit might be oxidation, perhaps from weathering, might also be the culprit.

NASA requires patching of pinholes larger than one millimeter. General Barry says investigators are trying to determine if the corrosion weakened one of the reinforced carbon panels along the left wing's front edge, known as an RCC panel. If so, it might explain how an RCC panel could be damaged by pieces of hard insulating foam that fell from the shuttle's external fuel tank during launch. Such damage is not proven, but has emerged as a leading possibility for the left wing breach.

"We're doing some more study," General Barry said. "We want to examine all of these issues with the pinholes as closely as we possibly can to try to get to the bottom of the question - the question being, if something hit the RCC and it was an aged RCC with pinholes or oxidation underneath that, would that be a contributing aspect to this mishap."

The shuttle accident investigators are planning tests in which fuel tank foam will be hurled at high speed at old and new RCC panels to see what damage they can cause.