NACOGDOCHES, Texas - Calls are still coming in to the sheriff's office here at the rate of about 75 an hour, calls from residents who think they have found another of the untold number of pieces of space shuttle Columbia.

There is an unprecedented search here for what have become puzzle pieces that investigators hope will provide the missing link to just why the space craft broke apart overhead Saturday morning.

The search for clues - some metal, some foam insulation, some of them the unspeakable remains of the seven-member crew - is taking place across a landscape of incredibly difficult terrain.

The 100-mile swath from the outskirts of Dallas into Louisiana includes four national forests, two of the region's largest bodies of water, and mile after mile that is uninhabited and inaccessible by paved roads.

"It's a searching nightmare," said Sheriff Tom Maddox of Sabine County, on the Louisiana border.

Teams traveled by horseback and four-wheel drive, by air and by foot. Four boats combed the Toledo Bend Reservoir using sonar to see beneath the surface. It was there that people reported seeing debris - by one account the size of a compact car - splash down Saturday. So far they have found nothing.

"If you ever dropped a penny in a bowl of water and watched what it does, this is what we are dealing with now," Maddox told reporters in the county seat of Hemphill.

The debris reported yesterday included the front of the space shuttle's nose cone, found buried deep in the ground near the Louisiana border, officials said last night.

"It's reasonably intact," said Warren Zahner, a senior coordinator for the Environmental Protection Agency, which is overseeing collection of shuttle debris.

Officials at the site where the nose cone was discovered described a hole about 20 feet wide in the pine forest. About 10 searchers emerged from the woods with bags full of debris, including metal objects. They filled a bed of a pickup truck with debris. A crew was to return to the site about three miles west of Hemphill today to excavate the nose cone.

Other large pieces found yesterday included a 7-foot piece of the crew cabin in Nacogdoches County, along with electronic parts believed to be part of the shuttle's flight control system, which Sheriff Thomas Kerss likened to the black box that records data in a conventional airliner. He declined to say exactly where they were found, nor would he name the place where they were taken.

In Sabine County, a large round piece of metal, maybe 4 feet in diameter, was spotted off the side of a country road waiting to be picked up.

Eventually the debris - which could ultimately be in the hundreds of thousands if not millions of pieces - will be taken to Barksdale Air Force Base in Shreveport, La. Officials hope to reconstruct Columbia either inside a hangar there or at another site.

That is why recovering every piece - no matter how small - is so important, said Ron Dittemore, NASA shuttle program manager.

"If we don't, it will make it very difficult to find the missing link," Dittemore said in Houston.

Much of the debris in Nacogdoches County sits just where it rained down Saturday. Officials didn't move much of it until yesterday, when EPA teams arrived.

Their first stops were the schools. Two small school districts were closed yesterday so students wouldn't share the playgrounds with potentially hazardous materials. Now the EPA will move on to the rest of the 1,200-plus pieces being cataloged here, and the agency is expected to bring in reinforcements today.

They're getting help from students at Stephen F. Austin State University, which is headquartered here. For years the school's researchers have worked with global positioning system technology and with Geographic Information Systems, or GIS, mapping software. In search of the shuttle, students have taken hand-held GPS devices and, working in connection with local officials, are documenting the longitude and latitude, as well as the size, shape and a description, of everything that has been discovered.

They will then create maps that they hope will provide information about the trajectory of the debris that may answer NASA's questions about what happened over Texas.