A small plane on a humanitarian mission from Southern California to Mexico crashed over the weekend, killing all four San Luis Obispo County residents who were onboard, including an attorney who spearheaded a major land conservation deal at Hearst Ranch near San Simeon.

The plane was piloted by attorney and conservationist Roger Lyon, who lived in Cayucos and was outside counsel to Hearst Corp. The passengers were Graciela Sarmiento and James Thornton, doctors from Arroyo Grande, and Andrew Thiel, a student at Cal Poly.

The four were on their way to San Quintin, a remote farming community about 90 miles south of Ensenada, as part of an all-volunteer organization called the Flying Samaritans. The Flying Sams, as they are known, were founded in 1961 to deliver free health care services to residents in Baja.

Lyon's 1973 Beechcraft Bonanza A-36 was reported missing Friday afternoon, said Victor Jones, president of the Flying Samaritans.

"It took off Friday at 2:40 p.m. from Ensenada, and Roger called back to the airport to say he was 10 miles south, which was normal procedure," Jones. "Then there was no radio contact."

The plane was reported missing Friday night, and Mexican search teams scoured the area. Rescuers found the wreckage Saturday, about 25 miles south of Ensenada, Jones said.

Jones said the cause of the crash has not been determined, but he doubted extreme weather was to blame. "There was a normal marine layer," he said.

Baja's civil protection director, Alfredo Escobedo, told the Associated Press that the plane apparently hit a 3,900-foot hill and then slid down to a mesa.

Lyon passed the bar in 1975 and had his own firm in Cayucos. He had volunteered with the Flying Samaritans since 1982, Jones said.

"Everyone donates their time and money," Jones said. "Pilots are flying their own planes, at their own expense. Passengers share the cost of fuel, and the costs of being there."

He said the group was heading to San Quintin to do reconstructive plastic surgery and provide other free medical help. "San Quintin is very impoverished, very remote. They are migrant farmers," Jones said. "They wouldn't be getting the help otherwise."

Stephen Hearst, vice president of the western properties division of Hearst Corp., which owns The Chronicle, had known Lyon for decades.

He said Lyon was the lead attorney who brokered the complex and acrimonious deal to preserve 82,000 acres of Central Coast landscape at Hearst Ranch in San Luis Obispo County from development. The deal was finalized in 2005.

"He was an extraordinary attorney and a wonderful guy. He was involved in land conservation before he started working on our deal at San Simeon," Hearst said.

"There was no more can-do, role-up-your-sleeves attorney than Roger Lyon," Hearst said. "He was a tremendous athlete and a wonderful family guy."