Federal prosecutors today charged two Texans with stealing debris that had showered down from the space shuttle Columbia and called on anyone else with wreckage to come forward by Thursday or face prosecution.

Merrie Hipp, 43, of Henderson, was charged with theft of government property for allegedly taking a shuttle circuit board she found Saturday. And Bradley Justin Gaudet, 23, a college student in Nacogdoches, was charged with stealing a piece of thermal insulating fabric.

They face as many as 10 years in prison if convicted. Both pleaded not guilty during court appearances this afternoon and were released on their own recognizance.

Many residents of this east Texas region have apparently taken wreckage as souvenirs; authorities said they had at least 17 investigations underway.

"These two individuals are first," said Matthew Orwig, U.S. attorney for Eastern Texas. "They are an example."

So many pieces have fallen here that local police have been unable to safeguard them all. But authorities said every piece is important.

"No one knows which piece will reveal the cause of this accident," said Michael Shelby, the U.S. attorney for southern Texas. "Every piece is potentially crucial to the investigation."

With that in mind, teams continued to meticulously search the waters, swamps and piney woods of east Texas and northwest Louisiana. Officials believed they were closing in on a large submerged object that could be the largest piece of debris from the Columbia space shuttle found so far.

Officials here said aerial surveillance and sonar readings taken over a portion of Toledo Bend Lake showed an object that appeared to be larger than 20 feet long resting in the water along the Texas and Louisiana borders. Dive teams with local and federal searchers were trying to reach the object this afternoon but postponed the effort because of cold and inclement weather.

The suspected piece of debris was located within the long swath of land and water where thousands of pieces of the shuttle and its components have been recovered, and along the path that the shuttle was expected to take toward a landing in Florida.

No one could say for sure what had been located, but some officials said it appeared to be larger than a car and speculated that it could be a crucial find because of its size and that it could help determine what happened to the shuttle as it disintegrated. NASA officials would not disclose the exact area they were searching, but it appeared they were focused on a small inlet where a local fisherman reported seeing a significant chunk of debris falling on Saturday.

"They are interested in any parts that can give them any clue," said Holly Morgan, a U.S. Forest Service spokeswoman. "The targets identified in the water appear to have a very high priority."

Investigators were also working on ways to remove the debris once it is pinpointed. A road was pushed through heavy woods so a truck could remove the shuttle's nose cone, which had been found late Monday. It was to be taken to Barksdale Air Force Base near Shreveport, La., where wreckage is being collected.

More than 600 people continued to search in Sabine County, Tex., and concentrated their efforts on a dense forest and a federal wilderness reserve north of this small town. Morgan said large amounts of debris remained in the woods and that important finds continue to be made. In the woods along the shuttle's path, investigators have found a piece of the shuttle's superstructure, part of the Columbia's landing gear, and pieces of what appears to be a control panel.

"There is still very high activity here," Morgan said. The small volunteer firehouse in the heart of Hemphill has been overrun by dozens of federal agents, state police and members of dozens of agencies. "It's a very labor intensive thing to do. We're literally fighting the underbrush."

Morgan said human remains continue to be recovered and are sent to Barksdale, about two hours away. A NASA spokesman at Barksdale said the remains of the astronauts were sent to Dover Air Force Base in Delaware today.

The Associated Press contributed to this report.

A Texas Forest Service worker prepares an area in the woods near Hemphill for removal of the Columbia's nose section, which crash-landed there. Merrie Hipp, 43, of Henderson, Tex., right, is escorted into the U.S. Courthouse in Tyler. She and Bradley Justin Gaudet, 23, of Nacogdoches, Tex., above, were accused of stealing government property -- debris from the Columbia shuttle.