Shane Krogen was to be lowered in a harness to a remote cleanup site in Sequoia National Forest when he fell Thursday morning, said Lt. Patrick Foy of the California Department of Fish and Wildlife.
"Everyone else had used it too," Foy said about the harness.
Krogen fell about 50 feet, investigators said. Foy was at the scene but did not see the fall.
The helicopter was flown by the California National Guard 129th Air Rescue Wing.
Krogen, 57, of Fresno was the founder and executive director of the High Sierra Trail Crew, a group that has worked with the fish and wildlife agency since 2008 and U.S. Forest Service since 1995 to remove trash and contaminants from illegal and remote marijuana gardens.
Krogen and some of his crew were among a handful of volunteers trained to be airlifted and lowered into difficult terrain.
"They were trained by the Department of Fish and Wildlife and the military," Foy said. "We just don't know what happened yet."
Fair weather and remote terrain have lured scores of people to establish illegal marijuana grow sites across the Sierra Nevada wilderness in recent years. They dam streams and spray pesticides and rodenticides, many that are banned in the U.S. They also leave behind tons of trash from campsites that are occupied during the five-month growing season.
Foy and about 15 other law enforcement agents had hiked to the grow site early Thursday. At about 10 a.m., Krogen and four other cleanup volunteers were to be transported by helicopter to a spot about 100 feet away.
"We could hear on the radio that the helicopter was coming in and lowering the crew members," Foy said. "Then a call went out that somebody had been injured."
Foy's team included at least three emergency medical technicians who were at Krogan's side within two minutes despite heavy brush and steep terrain. Krogen was alive, but his breathing was shallow.
The helicopter crew lowered a stretcher and hauled Krogen back up, then notified the trauma hospital in Visalia they were on the way.
"We all hiked out. Shane was breathing when we saw him. We all thought he was going to make it," Foy said.
His death caused an outpouring of grief among his friends, and tributes from those who worked alongside him.
"Shane's dedication to California's natural resources was extraordinary," fish and wildlife assistant chief John Baker said in a statement. "He and his crew have worked tirelessly for several years to maintain access to the high Sierra for all Californians."
In 2012 Krogen received the U.S. Forest Service's Regional Forester's Volunteer of the Year Award, and in 2011 he won the Chief's Award.
"He not only engaged in the reclamation of hundreds of marijuana sites, but also cleared trails after fires," the U.S. Forest Service said in a written statement. "His commitment and passion were evident in all the work he touched."
In July Krogen was quoted in an Associated Press story about the volunteers who help the U.S. Forest Service maintain trails during tight budget times.
"Most of us sit at a desk all week and at the end of the week have no tangible outcome for our effort," he said. "Whether they are cutting out a tree or building rock steps, they have a sense of pride and ownership."