Molten aluminum was found on Columbia's thermal tiles and inside the leading edge of the left wing, bolstering the theory that the shuttle was destroyed by hot gases that penetrated a damaged spot on the wing, the accident investigation board said Tuesday.

Roger Tetrault, a board member, said he suspects the melting of the spaceship's aluminum framework occurred because of the piercing gases and also because of the intense heat of falling through the atmosphere.

"My best guess would be that eventually we'll probably find both," Tetrault said. The melted aluminum, or slag, looks like black soot, and is present on both the right and left sides of the spacecraft, especially the left, he said.

"Many of the tiles on the left side have a thin, black deposit on them, and that deposit has never been seen on any previous flight," he said.

Tetrault said both tires from the left main landing gear also show evidence of extreme trauma from the Feb. 1 disaster: They are flat with torn fabric, possibly from a rupture in the final seconds of the spaceship's flight, Tetrault said.

He said the damage to the tires could also have been caused by heat penetrating the wing. That heat may have set off the small explosives that are used to free the landing gear if it gets stuck right before touchdown. The tire fibers may have been pulled apart by the heat of re-entry, he said.

"I would not speculate that it blew out the door or blew down the landing gear and that caused the accident. It is the result, not the initiating event," said Tetrault, a retired corporate executive with experience in nuclear submarines. He noted that rupturing tires "could have been the ultimate breakup event -- but we don't know that."

Investigators have theorized that foam or other debris that broke off the shuttle's big external fuel tank during liftoff Jan. 16 damaged the wing -- perhaps the leading edge, perhaps the area around the wheel well -- and allowed hot gases to penetrate the wing and destroy the shuttle. All seven astronauts were killed when their ship shattered over Texas, just minutes short of their planned Florida landing.

Tetrault would not speculate whether the wing's leading edge or wheel well was the location of that breach, but said both options were "equally alive."

"Everybody on the board has their own theory. I'm going to be patient and not express my theory at this time," he said.

What is particularly intriguing, Tetrault said, is that deposits of stainless steel were found along with molten aluminum on the inside of one of the carbon panels that protected the leading edge of the left wing. "How do you get that stainless steel and the aluminum up onto the back edge ... when in fact that stainless steel is behind the area," he said, adding that maybe it had something to do with the tires rupturing.

A sizable hole -- 4 inches by 2 inches -- was found burned into a left inboard elevon actuator, likely the result of the heat from re-entry, Tetrault said. Traces of hydraulic fluid that leaked from that hole surprisingly showed no significant overheating, he added.

More than 32,000 pounds of Columbia wreckage has been collected so far, representing about 13.7 percent of the returning spaceship. It is crucial to discover identical pieces from both sides of the ship for comparison, said the board's chairman, retired Navy Adm. Harold Gehman Jr.

"That's why the tires are so important," he said.

Investigators have recovered most of the pieces of the right landing gear door, for instance, but only three pieces of the frame of the left door and nothing from the door itself.

"We have more questions than answers right now. But we're getting smarter fast and I believe that there's a very good chance that we will, in fact, be able to localize the breach that occurred in the left wing," Tetrault said. "Until we have determined that location of the breach, every postulated cause of the accident is really just a theory."

Gehman, meanwhile, said the board will delve into what role NASA management and the agency's institutional culture played in the tragedy. But he said it is more important, for now, to find out what caused the accident.

He was referring to the flurry of e-mails among flight controllers and other engineers in the last few days of Columbia's flight, in which they discussed the possibility that the launch debris severely damaged the left wing. They said they were merely "what-iffing" and did not suspect any serious problems, even though some of them accurately predicted what might happen if a breach occurred.

"You've got to remember that at this point in the Challenger investigation, they knew what went wrong and so the review of who did what to whom and who did his job well and who didn't do a job well was relatively fairly focused," Gehman said. "I'm really not interested ... without any particular focus or without any particular reason, for just casting about and casting some big chill over NASA."

For the first time, the board's weekly news conference was not held at Johnson Space Center, but rather a few miles away, off NASA property. The panel wants to distance itself from the space agency and, in fact, has asked NASA's chief, Sean O'Keefe, to remove some top shuttle program officials from the investigation.

Gehman said he is satisfied that O'Keefe will comply with his request. He refused to name which NASA officials he wanted off the investigation.