Federal investigators say they found remains amid the wreckage of missing adventurer Steve Fossett's airplane in the mountains of eastern California, though there were conflicting reports on whether the remains were human.

The National Transportation Safety Board, or NTSB, said Thursday that searchers found enough at the crash site of Fossett's plane to provide coroners with DNA. They were found amid a field of debris that stretched 400 feet long and 150 feet wide in a steep section of the mountain range.

"We found human remains, but there's very little. Given the length of time the wreckage has been out there, it's not surprising there's not very much," said National Transportation Safety Board acting Chairman Mark Rosenker. "I'm not going to elaborate on what it is."

But Madera County Sheriff John Anderson later seemed to contradict Rosenker.

"We don't know if it's human. It certainly could be," Anderson said late Thursday. "I refuse to speculate."

Anderson told reporters earlier Thursday that searchers "found enough wreckage to determine that it was in fact the aircraft" Fossett was flying solo when he disappeared last September. It appears that Fossett plowed head-on into a mountainside.

"The crash looked to be so severe that I doubt if someone would have walked away from it," said the sheriff during a Thursday news conference before the body parts were found. The engine was lying about 300 feet from the wings and the fuselage, which disintegrated on impact.

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"It was a hard-impact crash, and he would've died instantly," said Jeff Page, emergency management coordinator for Lyon County, Nev., who assisted the search.

Fossett's mysterious disappearance prompted a massive search that covered 20,000 square miles — and sparked rumors that he faked his own death because he was buried in debt.

Crews confirmed about 11 p.m. Wednesday that the tail number matched the one on the single-engine Bellanca Fossett flew, according to Anderson.

The news came after federal transportation officials from the NTSB headed to California Thursday morning to retrieve the plane debris found in a mountainous, wooded area and join the investigation.

Rosenker, of the NTSB, said the agency has reviewed photographs of the site and confirmed "it appears to be consistent with a nonsurvivable accident."

The NTSB would bring in a private contractor to help with recovery of the airplane, Rosenker said. "It will take weeks, perhaps months, to get a better understanding of what happened," he said.

The discovery of the decimated fixed-wing plane came days after a local hiker stumbled upon some of Fossett's personal effects — including his pilot's license and FAA cards.

Anderson said the plane was found about a quarter-mile from where the items turned up, and authorities had not yet determined whether the sweatshirt that was with the documents belonged to 63-year-old Fossett.

In total, about 50 searches were planned in the coming days, according to the sheriff.

"We're certainly going to do an extensive search for remains," he said.

The National Transportation Safety Board went to analyze the remnants of the small plane and determine the cause of the crash after the overnight discovery of the debris by searchers not far from where hiker and local ski shop owner Preston Morrow came across Fossett's belongings.

The wreckage was located at about an elevation of 10,000 feet in the Sierra Nevada Mountains in the Mammoth Lakes, Calif., area, the NTSB said.

"I remember the day he crashed, there were large thunderheads over the peaks around us," Mono County Undersheriff Ralph Obenberger said, gesturing to the mountains flanking Mammoth Lakes.

Fossett — who made international headlines with a daring round-the-world balloon trek — was flying alone when he vanished Sept. 3, 2007 on what was supposed to be a routine local flight from Barron Hilton's private airstrip in Nevada and back. The plane was borrowed from the hotel magnate, and the flight was only supposed to last a few hours.

The initial aerial sighting was called in around sunset, according to Erica Stuart, spokeswoman for the Madera County Sheriff's Office.

Searchers had been combing a 10-mile radius around the spot where a hiker had found what appeared to be a pilot's license and other items belonging to Fossett in a thicket of woods not far from the Nevada state line.

Morrow told FOX News he was hiking alone with his dog near his home in Mammoth Lakes Monday afternoon when he came across what appeared to be three identification cards bearing Fossett's name and issued by the Federal Aviation Administration in Illinois.

Morrow, 43, also found $1,005 in cash. Anderson, the sheriff, said Thursday police assumed the money was Fossett's.

On Tuesday, Morrow returned to the site to get a GPS reading when he spotted a sweatshirt on top of a ridge.

Morrow then brought the items back home to his wife, a local fire captain, Mammoth Lakes Police Chief Randy Schienle told FOX News.

He then turned the items over to local police Wednesday after unsuccessful attempts to contact Fossett's family.

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Officers believe the cards are authentic, sources said.

The bills were tattered and crumpled on the ground; the weather-worn sweatshirt was nearby, Morrow told FOX. Both human and animal hair were found on the clothing.

Morrow said he didn't see any signs of Fossett's plane.

But his discovery prompted authorities to assemble a new search team to comb the area, Schienle said.

Fossett was the first person to ride the jet stream around the world in a balloon. He climbed some of the world's tallest and toughest mountains, sailed and set a number of world records.

On his last flight, he took off from Hilton's Nevada ranch.

The thrill-seeker never returned, and until now, searchers had found no trace of the plane — though authorities had believed it went down in a rugged region.

Fossett was declared legally dead in February.

Fossett, who had a house with his wife in Beaver Creek, Colo., made a fortune trading futures and options on Chicago markets.

In August, an attorney for Fossett's widow pleaded for an end to speculation circulating on the Internet that the millionaire balloonist and pilot may have faked his own death, possibly because he was heavily in debt.

FOX News' Catherine Donaldson-Evans, Allison McGevna, Adam Housley, Harris Faulkner and The Associated Press contributed to this report.