The wreckage of a small plane found in eastern California is the aircraft piloted by missing millionaire-adventurer Steve Fossett, officials said Thursday.

U.S. adventurer Steve Fossett, shown here in 2006, disappeared Sept. 3, 2007, after taking off from a Nevada ranch in a single-engine plane. (Kirsty Wigglesworth/Associated Press)

A five-centimetre-long piece of bone was found amid a field of debris, but there was no clear verdict from authorities on whether they had confirmed the remains were human.

"It's human remains," said Terry Williams, a spokesman with the U.S. National Transportation Safety Board. "We stick by that," he said.

"We don't know if it's human. It certainly could be," Madera County Sheriff John Anderson said later in the day. "I refuse to speculate."

The safety board's acting chairman, Mark Rosenker, wouldn't provide details about what searchers found. But he did say the remains are enough for a coroner to perform DNA testing,

Officials were able to confirm the aircraft belonged to Fossett by its N-number, which is used to identify aircraft, Anderson said.

Search teams began combing the rugged area around Mammoth Lakes for any sign of Fossett, 63, after a hiker found what appeared to be his pilot's licence and other personal belongings.

The renewed search spotted wreckage at an elevation of 3,048 metres in the Sierra Nevada Mountains about 400 metres from where the ID was found around sunset on Wednesday, according to a news release issued Thursday by the safety board.

Fossett has been missing since Sept. 3, 2007, after taking off in a single-engine Bellanca borrowed from a Nevada ranch owned by hotel magnate Barron Hilton.

Anderson said the crash was so severe that "I doubt someone would have walked away from it."

The plane appears to have crashed head-on with the mountainside before disintegrating, he said. The aircraft's engine was found about 90 metres from where the fuselage and wings were found.

Transportation board investigators will attempt to determine the cause of the crash, Anderson said.

Aviators had previously flown over Mammoth Lakes, about 145 kilometres south of the ranch Fossett took off from, but it had not been considered a likely place to find the missing aviator.

This year's biggest search for Fossett focused on Nevada's Wassuk Range, more than 80 kilometres north of Mammoth Lakes. That search ended last month.

ID found by hiker

These three pieces of identification belonging to Steve Fossett – a membership card from the Soaring Society of America, a U.S. Federal Aviation Administration card and a pilot's licence from the U.S. Department of Transportation – were found by a hiker on Monday in eastern California. (Rich Pedroncelli/Associated Press)A hiker, Preston Morrow, said he found a pilot's licence, a glider licence, a third piece of identification and $1,005 in cash tangled in a bush off a trail just west of the town of Mammoth Lakes on Monday.

Morrow, 43, who works in a local sporting goods store, said he initially didn't know who Fossett was. It wasn't until he showed the items to co-workers Tuesday that one of them recognized Fossett's name.

"It was just weird to find that much money in the backcountry, and the IDs," he said. "My immediate thought was it was a hiker or backpacker's stuff, and a bear got to the stuff and took it away to look for food or whatever."

Morrow told the Associated Press he turned the items over to local police following unsuccessful attempts to contact Fossett's family.

Fossett's name, address, date of birth and certificate number were included on the pilot's licence and matched Federal Aviation Administration records, said spokesman Ian Gregor.

Fossett was declared legally dead in February following a search that covered 32,000 square kilometres.

Wife hoped remains would be found

In a statement issued on Wednesday, Fossett's widow, Peggy, said she was aware of Morrow's discovery.

"I am hopeful that this search will locate the crash site and my husband's remains," she said. "I am grateful to all of those involved in this effort."

Fossett made a fortune trading futures and options on Chicago markets. He gained worldwide fame for more than 100 attempts and successes in setting records in high-tech balloons, gliders, jets and boats.

In 2002, he became the first person to circle the world solo in a balloon. He was inducted into the U.S. National Aviation Hall of Fame in July 2007.

Fossett also swam the English Channel, completed an ironman triathlon, competed in the Iditarod dog sled race and climbed some of the world's best-known mountain peaks.

With files from the Associated Press