Officials conflicted on whether they had confirmed the remains were human.
"We don't know if it's human. It certainly could be," Madera County Sheriff John Anderson said late Thursday, hours after the leader of the NTSB had said the remains were those of a person. "I refuse to speculate."
Fossett, the 63-year-old thrill-seeker, vanished on a solo flight 13 months ago. The mangled debris of his single-engine Bellanca was spotted from the air late Wednesday near the town of Mammoth Lakes and was identified by its tail number. Investigators said the plane had slammed straight into a mountainside.
"It was a hard-impact crash, and he would've died instantly," said Jeff Page, emergency management coordinator for Lyon County, Nev., who assisted in the search.
NTSB investigators went into the mountains Thursday to figure out what caused the plane to go down. Most of the fuselage disintegrated on impact, and the engine was found several hundred feet away at an elevation of 9,700 feet, authorities said.
"It will take weeks, perhaps months, to get a better understanding of what happened," Rosenker said before investigators set off.
Search crews and cadaver dogs scoured the steep terrain around the crash site in hopes of finding at least some trace of his body and solving the mystery of his disappearance once and for all. A sheriff's investigator found the 2-inch-long piece of bone. The remains are enough for a coroner to perform DNA testing, NTSB acting Chairman Mark Rosenker said.
"Given how long the wreckage has been out there, it's not surprising there's not very much," he said.
Fossett vanished on Sept. 3, 2007, after taking off from a Nevada ranch owned by hotel magnate Barron Hilton. The intrepid balloonist and pilot was scouting locations for an attempt to break the land speed record in a rocket-propelled car.
His disappearance spurred a huge search that covered 20,000 square miles, cost millions of dollars and included the use of infrared technology. Eventually, a judge declared Fossett legally dead in February. For a while, many of his friends held out hope he survived, given his many close scrapes with death over the years.
The breakthrough - in fact, the first trace of any kind - came earlier this week when a hiker stumbled across a pilot's license and other ID cards belonging to Fossett a quarter-mile from where the plane was later spotted in the Inyo National Forest. Investigators said animals might have dragged the IDs from the wreckage while picking over Fossett's remains.
The rugged area, situated about 65 miles from the ranch, had been flown over 19 times by the California Civil Air Patrol during the initial search, Anderson said. But it had not been considered a likely place to find the plane.
Lt. Col. Ronald Butts, a pilot who coordinated the Civil Air Patrol search effort, said gusty conditions along the mountains' upper elevations hampered efforts to search by air, as did the small amount of debris that remained after the plane crashed.
"Everything we could have done was done," Butts said. Searchers had concentrated on an area north of Mammoth Lakes, given what they knew about sightings of Fossett's plane, his travel plans and the amount of fuel he had.
"With it being an extremely mountainous area, it doesn't surprise me they had not found the aircraft there before," Lyon County Undersheriff Joe Sanford said.
As for what might have caused the wreck, Mono County, Calif., Undersheriff Ralph Obenberger said there were large storm clouds over the peaks around Mammoth Lakes on the day of the crash. Fossett made a fortune in the Chicago commodities market and gained worldwide fame for 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 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 peaks, including the Matterhorn in Switzerland and Mount Kilimanjaro in Tanzania.
"I hope now to be able to bring to closure a very painful chapter in my life," Fossett's widow, Peggy, said in a statement. "I prefer to think about Steve's life rather than his death and celebrate his many extraordinary accomplishments."
Marcus Wohlsen reported from San Francisco. Associated Press writers Malia Wollan in San Francisco and Scott Sonner in Reno, Nev., contributed to this report.