Search crews have covered almost all of the forested land that NASA has identified as the primary area where wreckage from the space shuttle Columbia likely fell in east Texas, but officials warned today that even the exhaustive search possibly could miss key components because the terrain is snaked with creeks and standing pools of water that are virtually unsearchable.

Officials in Sabine and Nacogdoches counties, where most of Columbia's wreckage landed after it broke apart over Texas on Feb. 1, said today that they had covered most of the targeted strip of land that stretches through rural east Texas and over the border into Louisiana. That search, so far, has turned up more than 6,000 bags of wreckage, and hundreds of pieces are being discovered daily.

But searchers have largely been stepping over and around the creeks and pooling water in the piney woods and swamps that cover the landscape. Such bodies of water account for about 1 percent of the search area in Sabine County.

Divers continue to enter the cold waters of the Toledo Bend Reservoir, searching almost 15 square miles where authorities believe significant pieces of the shuttle fell.

"We've been finding most of what they're looking for," said Bill Miller, a National Park Service spokesman in Hemphill, Tex. "But it may be impossible to get to some of it, if there are materials in the creeks. You can't find what you can't see."

Dry and warm conditions over the past three days have made it far easier for the hundreds of people still hacking their way through thick briars and underbrush.

The crews had contended with heavy rains, slippery forest floors and thick mud as three inches of rain fell last week. Forecasters are calling for as much as four inches of rain over the next three days.

Miller said it is easier now to find parts in the forest because it is winter. As spring moves closer, vegetation will sprout up with vigor, covering the ground and likely obscuring even obvious items.

"Once it dries out and gets warm, green will grow over everything," Miller said. "Then, it will be very difficult to find anything."

Late Tuesday, searchers on the eastern edge of the county found a ring, eight feet in diameter, that is believed to be a part of Columbia's engine. They also encountered unspecified hazardous materials associated with the wreckage. Searchers here discovered a mission patch that appeared to have come from an astronaut's uniform.

"It's unrealistic to think that we will be able to locate and collect every piece of the space shuttle in a short time frame," Nacogdoches Sheriff Thomas Kerss said. "We anticipate finding shuttle material for years to come."