JACKSON - John Walton was one of the world's richest men, an heir to the Wal-Mart fortune. But on Thursday, three days after he was killed in an aircraft accident, Walton's friends and family remembered him for his giving and his kindness, describing him as the "ultimate hero" to his wife and son.

Hundreds of mourners, including Walton's three siblings and mother and Wal-Mart executives, filed into the tree-shrouded Presbyterian church that Walton attended for a private memorial service.

"There was a great deal of sadness because it's truly a loss," the Rev. Paul Hayden, who also spoke during the service, said afterward. "But there was, in the midst of that, a great deal of gratitude, of thankfulness, of love."

Walton died Monday after the experimental, kit-built plane he was piloting crashed in a sagebrush flat near the Jackson Hole Airport in Grand Teton National Park. He was 58. Friends have described Walton as an experienced pilot who loved flying.

The National Transportation Safety Board is leading the investigation into the cause of the accident; an investigator arrived on the scene Tuesday and continued his work Thursday, NTSB spokesman Paul Schlamm said. A preliminary report could be issued as early as next week, he said, though a final accident report could take up to a year.

Walton's father, Sam Walton, founded Wal-Mart, a discount store chain that became one of the world's biggest companies. With an estimated net worth of $18.2 billion, John Walton was one of the world's wealthiest individuals.

But Marty Heires, a Wal-Mart spokesman, said Thursday that Walton never flaunted his wealth.

"He was a remarkable man," Heires said. "The thing you had to be impressed with was, he didn't have to do anything for anybody."

Friends this week described Walton as a man passionate about education, who was involved in, among other things, helping low-income students afford the cost of attending independent schools through a scholarship program and who volunteered at his son's school. That passion, and his philanthropy, were noted by speakers during the service, Hayden said.

Walton was also known for his love of the outdoors and adventures. One photo in the service bulletin shows him on a snowy peak, wearing a helmet, goggles and a slight smile.

"The place I think he really connected with his maker was nature," Hayden said outside the church, which sits beneath snow-capped mountains.

Walton's wife, Christy, in a poem, memorialized his adventurous streak and the way in which he'd regale his family with his tales: "saved to tell us the story, saved to try another day, saved to carry the edge, saved to carve the future, saved from death again and you finally lost the game."

An obituary provided by a family friend said the aircraft he crashed in "was an appropriate symbol of his love of life, zeal for adventure and passion for innovation." Walton, the obituary said, had recently flown the plane - described by the NTSB as a CGS Hawk Arrow - from West Virginia to Nebraska, but he was grounded by headwinds and called his brother, Jim, from a laundry.

"This spring, his mountain bike didn't receive as much attention as his new ultralight," the obituary said.