LOS ANGELES (AP) — Scot Alexander Breithaupt, who helped turn BMX bike racing from a backyard backwater into an international action sport, has died, authorities said.

Breithaupt was among the first to organize bicycle races on dirt motorcycle courses in the early 1970s, becoming first a founder of BMX — or bicycle motocross — then a champion, then one of its first famous faces.

“Scot was one of the key figures in making BMX become what it is today. He would say he was the key figure, because that was the kind of guy he was,” said Craig Barrette, spokesman for USA BMX, which runs the sport’s Hall of Fame, where Breithaupt is enshrined. “He was involved in every aspect of BMX.”

The sport, which later took on some of the same high-flying freestyle features as skateboarding, now draws crowds of thousands, fueled by energy-drink company sponsors and featured on ESPN’s X Games.

Among its biggest current stars is Jamie Bestwick, a 13-time X Games BMX gold medalist, who was part of a social media outpouring in the action sports world for Breithaupt.

“Sad to read about the passing of one of the all-time greats,” Bestwick said on his Twitter and Facebook pages. “Scot Breithaupt thank you for your amazing contributions and dedication to BMX.”

Another BMX Hall-of-Famer, Mike King, tweeted that it’s a “very sad day in the BMX world.”

Breithaupt’s death was unexpected, and the circumstances are murky.

Police responding to reports of a body near a shopping center in the desert city of Indio found him dead in a tent at a vacant lot, Sgt. Dan Marshall said Monday. Breithaupt, who was 57 and lived in neighboring La Quinta, had been dead for an unknown time, and there were no obvious signs of foul play, Marshall said. A cause of death had not been determined Monday.

Breithaupt was a teenager and a competitive motocross rider when one day he saw a group of kids riding their bicycles in a dirt lot near his home in Long Beach, Calif. He was inspired to organize bicycle races on a dirt track similar to those used by motocross riders.

“Those were some of the first BMX races ever,” Barrette said.

Breithaupt became a BMX rider, winning several championships.

He also became an early voice for the sport, introducing it to the nation as a color commentator in the early 1980s when it was televised on ESPN at a time when the network itself was new and specialized in novelties.

Later, he started manufacturing bikes, founding the company SE Racing and creating several innovative frame designs, Barrette said.

After retiring from active racing, he sold SE and started LM Productions, producing BMX and extreme-sport shows for ESPN and Fox