In the shuttle Columbia's final moments, the tires in the left wheel well blew out and at least some of the vehicle's steel and aluminum structure vaporized or melted, spraying molten droplets onto its heat-shielding tiles and inside the leading edge of the left wing, investigators said today.

These and other new findings have provided investigators with important new clues to the searing heat and violent forces that ripped the shuttle apart over Texas on Feb. 1. But they do not pinpoint what triggered the accident, Roger E. Tetrault and other members of the independent investigating board said at a briefing.

"What we have to do is follow the heat," Tetrault said.

Tetrault said the board learned Monday night that tests of the accumulating debris had revealed a black residue of aluminum oxide "never seen on any previous flight" on many of the recovered shuttle tiles from the left wing. There was also a "slag" of molten steel and aluminum inside the U-shaped carbon fiber heat shielding from the front of the wing. The deposits of melted metal apparently came from the strongest structural elements of the vehicle -- including the frames that support the wings. Stainless steel, he noted, melts at 2,500 degrees Fahrenheit.

The black residue and the droplets appear to be evidence of the extreme heat that may have consumed much of the shuttle's skin and frame, little of which have been found, the investigators said. They believe the process began when superheated gases flowed into the wing and wheel well through a breach in the wing area, and they are trying to work backward from the fragmentary evidence to pinpoint where the breach occurred.

The black residue, containing aluminum and hydraulic fluid, was found on heat-shielding tiles from both sides. The investigators hope that by comparing the debris from the left and right, they will be able to identify the effects of the original breach.

"Right now we have all these random pieces, and we're seeing all these marks and chars and destruction" that will yield more information when identical pieces from the right and left can be compared, said retired Adm. Harold W. Gehman Jr., chairman of the investigation.

Searchers have found all six of the shuttle's tires. The two from the left wing are torn and shredded from the apparent blowout, while the right side tires are relatively intact.

Whatever else was going on aboard the shuttle, Tetrault said, the bursting of the tires "would most likely have been a very catastrophic event." Telemetry has shown that the shuttle was under control for about half a minute after flight controllers at Johnson Space Center in Houston lost contact with the crew, so investigators believe the tires could not have blown before that.

"It's possible that the tires blew very late" in the chain of failure and destruction, Tetrault said.

Board member Steven B. Wallace, the FAA's chief of accident investigations, said the board is also focusing on the mission management team that presided over crucial decisions that may have contributed to the accident. Those include the assessment of potential damage to the shuttle's heat shielding by three pieces of foam or other debris that struck Columbia's left wing during launch, and the decision to "turn off" a request that the Defense Department use powerful telescopes and other equipment to try to assess any potential damage while in orbit.

"We will run all that to the ground," Wallace said.

Today's revelations underscored the importance of the recovered wreckage to the investigation. About 22,500 parts weighing 32,100 pounds, almost 14 percent of the orbiter by weight, have been recovered, and more than 16,000 parts have been identified. About 4,000 searchers are involved in the hunt, but they are mostly finding "bolts and screws," fragments of tile and other tiny bits.

The investigators are keenly interested in finding pieces that fell early in the shuttle's descent over California, Utah and Nevada, but have struck out. The recovered debris has been laid out in a large hangar at the Kennedy Space Center on a grid representing the shuttle. Tetrault said that as of last Thursday, the team had linked 105 containers of tile material to the left wing area. Because the serial numbers were burned off many of the tiles, investigators are struggling to put them together -- the shuttle was covered with 27,000 tiles -- based on size and shape.

A recovered left inboard elevon actuator -- a piece of the machinery that drove the vehicle's flight control surfaces -- was found to have a "sizable hole," 4 by 2 inches across, burned into it, Tetrault said.

Three pieces that appear to have come from the left landing gear door frame have also been found, but nothing else from the left landing gear door area has been positively identified.

The team has recovered fragments from 16 of the 22 pieces of carbon fiber material that coat the leading edge, Tetrault said. Tests on the part called Panel 9, which was located at a point of peak heating, showed the deposits of aluminum and stainless steel.

Among other things, Tetrault said, investigators are trying to find out how the molten metal was blown forward onto the inside of the leading edge.

Columbia Accident Investigation Board's Roger E. Tetrault holds up two images of the shuttle Columbia's main landing gear tires at a news conference yesterday.