With rain clouds looming overhead and an area the size of West Virginia left to scour, authorities continued their scavenger hunt on Monday for debris and human remains from the space shuttle Columbia.

Hundreds of investigators returned to the forests of East Texas not only to hunt for more debris but also for the remains of the shuttle's seven astronauts.

Searchers with expertise in airline accidents, engineering and forensics flocked to Texas and Louisiana to help pick up any wreckage that may give them clues as to what happened in those final minutes before the space shuttle broke apart over Texas 16 minutes before it was scheduled to land in Florida Saturday.

• Map: Debris Sightings

NASA also expanded the search on Monday, establishing a second collection site at Carswell Air Force Base near Fort Worth.

"It turns out that the debris field is quite large and still really being determined," said retired Air Force Gen. Mike Kostelnik, a deputy associate administrator for NASA. "Today we find there is more things further west than we anticipated."

Meanwhile, divers headed to the depths of a huge reservoir where part of the shuttle may have splashed down.

Among the items discovered so far: a car-size chunk that splashed into Toledo Bend Reservoir on the Texas-Louisiana state line, a 7- to 8-foot door-like fragment, what resembles part of a windshield and a 5- to 6-foot-long object authorities suspect could be part of the landing gear.

There have been more grim discoveries -- human remains, including a leg, torso, thigh bone and skull. NASA confirmed the remains of some of the seven Columbia astronauts had been recovered.

In San Augustine, just east of Nacogdoches, Larry Epps put a 55-gallon barrel over a piece of metal that landed in his hay meadow to protect it.

"If it hit me, my wife would have been a widow," he said of the hollow gray object that resembles a tire. He later found what appeared to be a circuit board about 100 yards away from his front yard and a half dozen 2-by-2-inch metal pieces in his meadow.

Meanwhile, global positioning satellite technicians went back to work at the crack of dawn Monday to log the location of debris, said James Kroll, director of the Emergency Geospatial Mapping Center at Stephen F. Austin State University in Nacogdoches.

"We're still out there. We've got 15 teams mapping stuff all over, if they ever decide to come pick it up," Kroll said.

The maps will help investigators retrieve pieces of a puzzle that may help them determine just what went wrong.

Local authorities, overwhelmed by the enormity of their tasks, scrambled to locate and guard objects in the sprawling debris field, as NASA established command posts in Lufkin, Texas and at Barksdale Air Force Base in Louisiana to oversee recovery and examination of the wreckage.

With classes canceled at schools in Douglass, Texas Monday, school Principal Jay Tullos waited for experts to collect the 25 pieces of debris that had fallen on the grounds of his school.

"They said they'd be here first thing in the morning. We're waiting on a call," Tullos said.

About 300 people from 30 agencies -- including the FBI, Federal Emergency Management Agency, National Transportation Safety Board and the Texas Department of Public Safety -- were being assigned to collect thousands of pieces as small as pebbles and as big as pickup trucks.

The wreckage will be trucked to Barksdale, where engineers from shuttle contractor United Space Alliance will sort through it in search of clues.

Debris and human remains started arriving at Barksdale "in everything from helicopters to rental cars," NASA spokesman Steve Nesbitt said.

"Most of the things are not very large, from the size of a trash can lid to something much smaller, about half the size of a baseball bat," Nesbitt said in a telephone interview from the base.

New evidence released Sunday shows that the temperature on Columbia's left side shot up and the ship was buffeted by greater wind resistance before it disintegrated over Texas.

FEMA is staffing up Disaster Field Offices in Lufkin and at Barksdale to coordinate the "search find and secure" mission. A FEMA spokesman told Fox News that searchers were making high-concentration debris fields a high priority with special attention given to those near schools.

There were about 15 hazmat teams from the federal Environmental Protection Agency in Texas. That number will jump to 30 on Tuesday. Teams will assess the nature of the debris and whether or not it’s toxic.

A specially trained Coast Guard team, called a "gulf strike team," which is based in Mobile, Ala., has been deployed. Team members have special training to deal with oil and hazardous materials.

The salvage operation covers an area that stretches from the rolling hills of East Texas to a suburb of New Orleans, where authorities found what could be insulation from Columbia.

Louisiana state police said they had at least 20 possible shuttle pieces in 10 parishes as of Monday morning.

Fourteen divers from the Texas Department of Public Safety recovery team prepared Monday to search the Toledo Bend Reservoir, where fishermen said they saw a car-size chunk splash into the water Saturday.

The divers would wear helmets equipped with cameras, said dive team commander Lynn Dixon.

"We're not anticipating any visibility. The camera sees more than divers will see," Dixon said. Divers would have to rely on sense of touch to get around.

The search for wreckage so far has focused on Texas, where Gov. Rick Perry said 33 counties -- a 28,000-square-mile area from north of Dallas all the way to the Gulf Coast -- had reported debris. That amounts to about 10 percent of Texas' land area, or more than all of West Virginia.

One emphasis, Kostelnik said, is the recovery of the remains of the seven astronauts, mostly in the Lufkin area.

"We are trying to recover these national heroes and get them back to their families as soon as possible," Kostelnik said.

The heart of the operation is East Texas, a region of thick forests of pines and oaks, expansive farm land and cow pastures. It holds four national forests, covering almost 700,000 acres, and two reservoirs that together span about 300,000 acres. The thick woods also are home to wild hogs and bobcats.

While the region is a magnet for hunters, boaters and anglers, its challenging terrain has been giving Columbia searchers difficulty.

"This is forest -- dense forest," said Kroll. "There is no way to describe how many pieces there are and how spread over the landscape they are.

"Ten years from now, folks are going to be walking around the woods and finding stuff."

In Nacogdoches County alone, authorities have logged more than 1,200 confirmed debris sites. State troopers and local authorities didn't have enough personnel to protect every piece, but they manned 130 spots -- alongside two-lane highways, restaurants and ranches -- to guard debris against scavengers.

They said NASA had provided a list of priorities: anything that could contain data or resembles computer circuitry, or potentially radioactive materials.

Marc Masferrer, editor of The Lufkin Daily News, said a landowner led him to what appeared to be a seat from the shuttle in a pasture 20 miles west of Nacogdoches.

Fox News' Catherine Herridge and The Associated Press contributed to this report.