-- Investigators Friday found a two-foot fragment of a wing of the destroyed space shuttle Columbia near Fort Worth, a piece of wreckage they hoped might yield clues to crack open the mystery of the shuttle's disintegration one week ago.

Officials said the fragment included a portion of the wing's leading edge, but they had not yet identified which wing it came from. Possible damage to the leading edge of the left wing is one focus of the inquiry into the chain of events that led to the shuttle's fiery demise.

NASA officials also released a fuzzy black-and-white photo of the shuttle as it flew over New Mexico. Although grainy and somewhat unfocused, it shows what appears to be a trail behind the left wing and a slight protrusion along its leading edge.

The picture, taken by an Air Force tracking camera at Kirtland Air Force Base in Albuquerque, was considered potentially important because it was taken as telemetry monitored at Mission Control showed sensors in the left wing suddenly shutting down. At the time, the crew reported no problems and the spaceship appeared to be flying normally through its reentry into Earth's atmosphere.

Seconds after the picture was taken, communication with the spacecraft was lost and it disintegrated over Texas.

Shuttle program manager Ronald D. Dittemore said investigators are trying to obtain pictures of previous missions taken from the same location to compare with the new image, to determine whether there is something unusual about the picture.

"These things are not black and white," he told reporters. "You can have a particular photo and someone may draw immediate conclusions, and we don't draw the same conclusions."

The wing wreckage was the first potentially significant piece of evidence of the type NASA investigators have said they need to determine why the Columbia came apart. Officials have been especially keen to recover pieces of the wing, and Michael Kostelnik, NASA's deputy associate administrator, described the find as "significant."

Kostelnik said the fragment included a portion of the black, reinforced carbon-carbon composite that forms the leading edge of the wing and is able to withstand temperatures of up to 3,000 degrees Fahrenheit during reentry. Dittemore said the fragment measured 26 inches by 12 inches by 7 inches -- about the size of a medium-size travel suitcase.

The shuttle program manager said 18 inches of the wing structure was still attached, and Kostelnik indicated that a section of insulating tile might also be intact. Each of the 24,000 tiles on the shuttle is numbered, which means that investigators should quickly be able to determine where the fragment came from. It was being taken Friday to the former Carswell Air Force Base outside Dallas, one of two collection sites for the widely scattered wreckage.

Officials at Mission Control reiterated Friday that there was no warning that anything was wrong as Columbia crossed the California coastline one week ago.

While some video footage taken by observers appears to suggest that the shuttle may have been shedding fragments at that point, no confirmed wreckage from Columbia has been found west of Texas, NASA officials said. The crew was apparently unaware of serious trouble until nearly 9 a.m. EST, when the shuttle was over Texas and the last radio transmission from the crew abruptly terminated.

Reviewing the timeline that investigators cobbled together of Columbia's final minutes, Dittemore said the earliest sign of a problem came around 20 seconds after 8:52 a.m. EST, when a sensor in the left main gear brake line in the wing showed a temperature increase of two degrees per minute. Nineteen seconds later, another sensor started showing a rise of six degrees per minute. At 8:52:59 a.m., a left inboard elevon -- a movable flap on the rear of the wing -- stopped working.

Dittemore described a relentless series of sensor readings showing temperature increases or abrupt failures in the seconds that followed -- a steady progression in the wheel well, brake lines and elevons. Another elevon stopped working at 8:53:36; an interior sensor started showing a six-degree-per-minute temperature rise at 8:54:22. Several sensors stopped working after 8:58, even as the shuttle appeared to be streaking toward Texas with no obvious flight problems.

Dittemore said it is clear "something is happening earlier in Arizona and New Mexico. We need to understand what that is trying to tell us. But we don't see anything manifested in the data or flying qualities of the vehicle. That is very interesting."

One challenge facing investigators is to explain the long delay between the first sign of trouble with the sensors and the break-off in communication eight minutes later -- a very long time for a craft traveling faster than 12,500 miles per hour. It means that whatever doomed Columbia gradually unfolded over something like a thousand miles.

Meanwhile, teams of NASA officials continued to fan out across the west to evaluate reports of debris. There have been 350 reports outside Texas, including more than a hundred each in California and Arizona.

Officials said they were poring over thousands of pictures, but were hampered by concerns that some had been touched up or modified.

Debris teams focused most intensively on the area around Fort Worth, and Kostelnik said searchers were concentrating on a region 150 miles west and northwest of the city. About 1,200 local officials, NASA personnel, National Guardsmen and state troopers were taking part in the search, using dogs and horses wherever needed, he said.

Members of the Columbia Accident Investigation Board, which is taking over the probe, arrived at various NASA facilities, led by Adm. Harold W. Gehman Jr. Officials at NASA said the independence of the board was a guarantee that the public would be kept informed about the course of the investigation -- in contrast to the complaints of secretiveness that dogged the agency's initial response to the shuttle Challenger disaster in 1986.

In a closed-door meeting with employees at the Johnson Space Center here, NASA Administrator Sean O'Keefe, wearing an open-neck T-shirt, referred pointedly to the intense media pressure on the agency.

He referred to "paparazzi" and joked that state troopers were eager to hustle reporters away. "They wake up in the middle of the night wanting to do this," he said, to laughter from the employees.

The NASA administrator also criticized some reporters for pursuing officials for individual interviews instead of relying on official news briefings.

O'Keefe tried to reassure employees that the investigation was not looking for scapegoats. "We are all responsible," he said, adding that "as we work through this, we are going to be fair. . . . We all must stick together."

The agency chief voiced strong support for the investigation board, which he activated hours after the crash. The panel is being expanded with new members, who have not yet been named, in response to criticism that it is dominated by military and NASA officials.

He said the decision to create the board was a direct outcome of the Challenger disaster, and that its presence would add credibility to the investigation and "take us out of the sensitive position" of judging their own performance.

"They are part of this solution," he said. He added later, "Nonetheless, they are independent."

This Air Force photo released Friday by NASA appears to show a trail behind the left wing of the Columbia about one minute before it disintegrated. NASA official Michael Kostelnik called the discovery of a wing fragment from the Columbia "significant." He said intensive searches are being conducted west and northwest of Fort Worth for more wreckage.