Search crews and investigators were withdrawn from the sprawling wilderness here this afternoon as heavy and chilling rains softened the terrain and created a high risk of hypothermia, slowing the search for the scattered wreckage from the space shuttle Columbia.

Officials here and across the region said the weather was impeding progress in several areas, preventing aerial surveillance and making the dense forests a hazard to searchers on foot.

The rain also could spell trouble for debris that has already been tagged and left in the field for collection. "Large amounts of rain can cause flash flooding, and potential runoff could affect known debris areas," said Dave Bary, a spokesman for the Environmental Protection Agency who is working in Lufkin, Tex.

The cold rain was expected to continue into the weekend, and some forecasts were calling for light snow across east Texas and northwest Louisiana on Saturday. Officials said any snow accumulation could complicate the search for pieces of the shuttle and hamper efforts to collect evidence that could be vital to determining what caused it to disintegrate.

Search crews on horseback, focusing on the protected federal wilderness areas in Texas, were held back today. Federal and local dive teams tried to enter Toledo Bend Lake, where they expected to find wreckage from last Saturday's disaster, but wind and waves frustrated their efforts.

"The weather is kicking our rear ends bad," said Sheriff Billy Rowles of Jasper, who is in charge of the water recovery effort. "We really need a couple of days of good weather."

The lake, which is on the Texas-Louisiana border, has been a prime target for NASA officials because it lies directly in the debris field and could hold critical evidence of Columbia's disintegration. U.S. Forest Service spotters who patrolled a stretch of the lake earlier this week said they saw at least three distinct items under water, one of which appeared to be more than 20 feet long and was not far from shore.

Local, state and federal officials said that if the object turns out to be a part of the Columbia, it would be the largest piece discovered so far.

NASA is also focusing on recovering the "red tag" items that could help explain why the shuttle broke apart as it reentered Earth's atmosphere last weekend. Bary said electronic components continue to be the highest priority, though he said there were no reports of significant finds in the field today.

The delays in collecting wreckage also slowed the flow of items to Barksdale Air Force Base near Shreveport, La., where investigators are cataloguing items and collecting data and hope to ultimately begin to put the pieces together in a large airplane hangar, said Paul Foreman, a NASA spokesman there.

National Guard members and members of an evidence recovery team today continued to try to pull more than 600 pieces of previously discovered wreckage from the piney woods in Sabine County, Tex. Jamie Gunter, operations chief in Hemphill, said some of the pieces disappeared after they were marked, and law enforcement officials believe some of them were looted.

Matthew Orwig, U.S. attorney for the Eastern District of Texas, said more people had called to report having wreckage or had taken wreckage to local sheriff's offices since he announced that they could avoid charges for a short period. That "leniency" period came after two people had been indicted on charges of stealing government property from the wreckage.

Orwig said that several people had also called to report that other people have wreckage , and that authorities were investigating those claims.

A team searched for shuttle wreckage in the Sabine National Forest near Hemphill, Tex., where inclement weather has been hampering efforts.