Rain-drenched searchers trudging through the muddy forests and fields of East Texas gathered everything from tiny slivers to huge chunks of debris Thursday as they tried to find clues to what caused the space shuttle Columbia's destruction.

Investigators also checked reports of debris in California and Arizona, but shuttle program manager Ron Dittemore said late Thursday that as far as he knew, no shuttle parts had been confirmed west of Fort Worth, Texas.

"We're still looking for that elusive missing link," he said.

So far, none of the more than 12,000 individual pieces found in a field stretching across East Texas and Louisiana has provided the critical answers NASA is looking for.

The shuttle was composed of about 2 million parts, many of which shattered into pieces as small as a nickel. The key pieces for NASA's investigation will be the data recorders, certain tiles and parts from the left wing where sensors showed a temperature rise before the shuttle broke up over Texas on Saturday.

With the heavy rain falling throughout the day across the debris field Thursday, experts worried that sensitive pieces could end up buried in mud or degrade if left exposed to the rain.

"Obviously the weather is a significant factor for us, but we are continuing on," Nacogdoches County Judge Sue Kennedy said.

Clad in long yellow raincoats and cowboy hats with protective covering, volunteers and NASA-trained searchers scoured the ground in East Texas for even the tiniest pieces. Sabine County Sheriff Tommy Maddox said people were bringing in everything from tiny items to pieces 15 to 20 feet long.

At the Toledo Bend Reservoir along the Louisiana border, meanwhile, searchers in Army helicopters said they had spotted at least one large object in the water. EPA senior response manager Jim Mullins said it could be the car-size piece of debris reported by fishermen Saturday.

A dive team was unable to begin hunting through the reservoir Thursday, but the team was standing by with an underwater remote-operated vehicle equipped with cameras and sonar.

David Bary, an Environmental Protection Agency spokesman, said searchers had stopped counting the individual pieces found at about 12,000. The parts include the shuttle's nose cone and at least two possible wing sections, though it wasn't clear which side of the shuttle they came from.

"We're simply consolidating the material and preparing it for shipment to Barksdale Air Force Base," said Bary, whose agency oversees collection of the material.

The debris, including about 1,000 pieces already on the way to Barksdale, will be "laid out for viewing and inspection" by officials, then taken to Kennedy Space Center to be reconstructed, Dittemore said.

A final resting place for the Columbia hasn't been decided, he said. The Challenger debris is buried in a pair of abandoned missile silos at the Cape Canaveral Air Force Station, adjacent to Kennedy Space Center.

Also Thursday, authorities said several people had turned in items since federal prosecutors announced an amnesty period for anyone who illegally picked up Columbia debris. They did not provide numbers or details.

On Wednesday, two people had been charged with stealing government property -- a circuit board and thermal insulating fabric from the shuttle.