Following tantalizing clues, including molten aluminum that splashed across the Columbia during its final seconds, investigators said Tuesday that they are beginning to zero in on the precise spot where superheated gas breached the craft and brought it down.
The clues appear to support the notion that superheated "plasma" gas penetrated the craft's left side, members of the Columbia Accident Investigation Board said at a news conference.
Among other discoveries made in a whirlwind week of investigation, board member Roger Tetrault told reporters that officials have detected traces of molten aluminum on a surprisingly large array of debris recovered on the ground. Tetrault, a retired corporate executive who once built nuclear submarines, said the substance has even been found on debris that came from the right side of the craft -- far from the area on the left wing where the breach is believed to have occurred.
The substance is believed to be the melted aluminum skin of the shuttle, officials said.
Investigators have also recovered all six of Columbia's wheels and have determined that the left wheels suffered extreme trauma during the accident.
The tires were housed in the left wheel well, one of the areas investigators say was likely struck by foam debris that fell from an external tank and hit the shuttle during its Jan. 16 launch.
Investigators say that while they are continuing to investigate many avenues, they have not been presented with any evidence that the shuttle was brought down by anything other than the launch accident.
Tetrault said the board is looking at whether the tires, which cost $100,000 apiece and were made by Michelin, blew up after some sort of "catastrophic event" -- possibly because of heat that had penetrated the left wing. The heat, officials said, might have set off the miniature explosives used to push down the landing gear.
Many of the space shuttle's heat-resistant tiles recovered on the ground in Texas are covered with a sooty film that has never been seen before on a space shuttle that has returned safely, Tetrault said. And a 4-by-2-inch hole was found burned into a left elevon actuator, a device at the rear of the wing used for flight control. But officials cautioned that the damage to the device was likely caused after the piece dislodged from the craft and tumbled through the atmosphere.
"We have more questions than answers," Tetrault said. "But we are getting smarter fast.... Until we have determined that location of the breach, every postulated cause of the accident is really just a theory."
Also Tuesday, retired Adm. Harold W. Gehman Jr., chairman of the investigative board, attempted to dispel any lingering concerns raised by a brief tiff with NASA over the board's independence.
Most of the board's members have extensive ties to the military and the aerospace community, and critics have questioned whether the board could conduct an investigation independent of NASA.
Gehman has attempted to distance himself from NASA by, among other things, hiring his own experts to provide technical advice. Gehman also held Tuesday's news conference outside the grounds of NASA's Johnson Space Center for the first time, largely to demonstrate the panel's independence, aides said.
Gehman recently asked NASA Administrator Sean O'Keefe to remove some space shuttle program officials from the investigation. O'Keefe bristled at the suggestion, and some within NASA were concerned that the board's investigation could degenerate into a witch hunt.
But Gehman said Tuesday that he asked for the personnel moves because he could not stomach having "investigators investigating themselves," and O'Keefe has agreed to the changes.
"You can color me satisfied," Gehman said.
Still, he has pledged to not only search for a cause of the accident but also to look at the culture of NASA. A series of internal e-mails released in the last two weeks show that more than a dozen NASA engineers questioned whether the Columbia had been badly damaged by the liftoff accident. Word of those concerns did not reach top NASA administrators until after the accident, though some of the engineers predicted precisely the scenario that is believed to have caused the loss of the shuttle.
At the space agency, the investigation remains an extremely sensitive matter.
NASA officials, speaking with reporters in Washington on Tuesday, reiterated their view that internal debate over the damage caused by the liftoff accident should be viewed as a routine "what-if" exercise, not a symptom of recalcitrance or poor communications.
O'Keefe said one participant in the e-mail debate told him during a meeting last week at NASA's Langley Research Center in Virginia that the exchange among engineers had been misconstrued by the media.
"This is being characterized as if we were waving red flags," the participant said, according to O'Keefe. "Nothing could be further from the truth."
Pressed for more details, O'Keefe declined to elaborate.
"No, no," he said. "It would be another log on the fire."
Survivors of the seven crew members killed in the Columbia accident thanked the world Tuesday for a "generous outpouring of support and affection." The families announced that several charitable funds have been established on their behalf. Information about the funds can be found at www.columbiashuttlefund.com.
Gold reported from Houston, Anderson from Washington. Times staff writer Ralph Vartabedian contributed to this report.