NASA said today that the shuttle Columbia may have begun to break up as it entered Earth's atmosphere over California, and it dispatched teams to search there and in Arizona for wreckage.

The pieces that fell first from the doomed spacecraft are "extremely important" because they could "shed important light" on the cause of Saturday's disaster, which killed the seven astronauts on board, Michael D. Kostelnik, deputy associate administrator for the international space station and the space shuttle, said at a Washington briefing.

Investigators believe the first pieces shed from the shuttle are probably closest to the first source of trouble.

Since the spacecraft disintegrated, searchers have concentrated on east Texas, where thousands of pieces of the shuttle rained down and the remains of some of the astronauts also have been discovered. Local officials here said they found what they believed to be a seven-foot section of the shuttle's wing in a pond. Major sections of the shuttle, including the nose cone, pieces of the cabin and engine parts, have been discovered near here in recent days as search efforts have intensified with the use of helicopters, horses, boats and hundreds of foot patrols. At least 15 astronauts joined them today.

An independent investigative panel, led by retired Adm. Harold W. Gehman Jr., flew to Nacogdoches today to see the wreckage.

"Our imperative is to get it right," Gehman said. "The astronauts who will fly in future orbiter missions need to know we have done everything we possibly can to come to the bottom of this and fix it."

NASA's new interest in California and Arizona came after Anthony Beasley, a California Institute of Technology astronomer, said he saw the shuttle surrounded by several points of light as it crossed the Sierra Nevada Mountains. "I knew that shuttles sometimes leave tiles when they reenter, and that's what it looked like," he said in an interview.

Officials cautioned that it is possible that Beasley saw the normal plasma burnoff that comes with reentry, but NASA is taking the report seriously enough to investigate it.

A spokesman for the California Highway Patrol said NASA told state officials it was sending two teams to the state to examine suspected wreckage and had asked that the locations not be disclosed.

County officials in Texas, meanwhile, began collecting material in plastic bags and buckets today, and they looked for secure places to store it until federal officials could take it to Barksdale Air Force Base in Louisiana. Search leaders had complained in recent days that material increasingly was disappearing as they waited for EPA teams to collect items that had been marked for days with police tape or orange flags.

"People are starting to get souvenirs, you know what I mean?" said San Augustine County Judge B. Wayne Holt, head of emergency efforts there.

Nacogdoches County officials said that on Wednesday morning they expect federal officials to issue at least one arrest warrant in connection with thefts of wreckage.

Federal officials are overseeing human remains and major pieces of the shuttle.

Holt said he expected federal officials to pick up the wreckage warehoused in San Augustine using a large truck. More than 2,000 wreckage sites have been identified in the county, including one stretch where "it's almost a solid run for miles," he said.

The nose cone, which was discovered Monday outside Hemphill near the Louisiana border, was wrapped in plastic and tape overnight. It is about three feet high and five feet wide; officials said a helicopter would transfer it to Barksdale.

The nose cone was found by two men who live near its landing site, about 100 yards from a main road through rural Sabine County. Tim "Peewee" Mitchell, 33, a building contractor, was searching for wreckage with his nephew when he saw an area that looked disturbed, with branches down and mud splashed on trees.

"When we first saw it, we just thought it was a garbage pile," he said.

But he soon saw the cone, planted sideways in the mud, jutting several feet into the air. He and neighbor Nathan Ener, 40, saw numbers and letters marking the cone. Wiring in red, white, blue and green was visible.

"It wasn't burnt at all," said Ener, who alerted the sheriff's office. "There were little pieces of everything all over the place."

Ener said he also found a round window that was two feet wide and what appeared to be an engine component that was six feet high and eight feet wide.

At Barksdale, federal officials continued to receive significant amounts of wreckage, gathering it in an aircraft hangar as investigators began planning what to do with the pieces of the shuttle. They are carefully cataloguing what has been found as wreckage arrives by truck and helicopter from Texas and Louisiana.

"Anything you can imagine is coming in," NASA spokesman Steve Nesbitt said. "It's all sorts of things, from electronics to small parts to chunks of metal. The cumulative amount of debris coming in here is beginning to build."

Members of the independent board are meeting this week to decide how best to proceed with the investigation, but it is likely that the wreckage will be laid out in a 15,000-square-foot hangar at the heart of the base in a building that is larger than two football fields and usually provides a work area for B-52 bombers, he said. Investigators may try to piece together segments of the shuttle.

Nesbitt said the investigation will be similar to those conducted by the National Transportation Safety Board, which tries to reconstruct airplanes after crashes. The wreckage from Columbia may seem too charred to be valuable, but experienced investigators say it can yield plenty of clues.

"Despite the fact that it was burned to hell and beat to hell, it will tell them something," said Bernard Loeb, former aviation safety chief for the NTSB who is not involved in the shuttle inquiry but has investigated airliner crashes.

Six safety board experts are working in the field with NASA investigators. Two of them have intimate knowledge of in-flight breakups and fires. Investigators Bob Benzon and Frank Hilldrup were among the chief investigators at the 1996 explosion and crash of Trans World Airlines Flight 800.

Loeb said patterns of thermal damage can offer vital clues about whether a craft broke apart at the time of the original event that brought it down, broke apart on the way down or was fractured as it hit the ground.

White reported from Bossier City, La. Also contributing to this report were staff writers Don Phillips in Washington and Manuel Roig-Franzia at Cape Canaveral.

A piece of wreckage believed to be a Columbia window frame, still containing glass, lies in a swampy area close to where the nose cone was found near Hemphill, Tex. The search for wreckage has intensified.

Above, members of the Texas Dive Recovery Team lower side-scan sonar equipment into the Toledo Bend Reservoir yesterday near Hemphill, Tex. The sonar, which bounces sound waves off the bottom of the reservoir, helped locate a car-size piece of what authorities believe is shuttle wreckage.

A soldier moves flowers and American flags as he recovers a piece of wreckage near Nacogdoches, Tex. About 12,000 pieces had been collected by Monday.