A hunter who lost control of an illegal campfire ignited the massive blaze on the west edge of Yosemite National Park, authorities said Thursday.

The announcement, after a two-week investigation by the U.S. Forest Service, puts the devastating Rim Fire among a long list of California wildfires blamed on human actions that were - if perhaps ill-considered, untimely or reckless - purely accidental.

The fate of the latest alleged fire-starter, though, remains unknown. Officials declined to release the name of the person, who has not been charged, or detail exactly what happened, saying the investigation is continuing.

They said the hunter started a campfire while traveling in remote wilderness within the Stanislaus National Forest, about 3 miles east of the Sierra foothill town of Groveland.

Campfires are prohibited in the summer months in the forest because of high fire danger.

The campfire turned into a wildfire Aug. 17 and has since burned more than 370 square miles, both in the national forest and nearby Yosemite. By Thursday, the fire had become the fourth largest in California history and had destroyed 111 structures, including 11 homes.

80 percent contained

The cost of fighting the fire, which was 80 percent contained as of Thursday afternoon, stood at nearly $80 million.

Penalties for starting a damaging wildfire vary, but criminal convictions have historically brought a range of punishments, from fines to community service to prison time - even when the ignition was accidental.

The cases often turn on whether the fire-starter was deemed to be reckless or irresponsible.

The 2003 Cedar Fire east of San Diego, which killed 14 people and burned 2,200 homes, was blamed on a deer hunter who was lost and ignited a fire to make his whereabouts known. Sergio Martinez pleaded guilty and served six months in a halfway house.

Matt Rupp, who was accused of starting the Bear Fire near Redding in 2004 by mowing dry grass, took his chances at trial and got four years in prison. The Bear fire gutted 86 homes, and victims were angered by testimony that Rupp had ignored warnings not to mow, telling one passerby, "Go to hell."

But in 2008, after a fire burned 30 homes outside Yosemite, a target shooter who shot steel-jacketed bullets along a riverbank was given only probation and community service.

Ray Mooney, a spokesman for the U.S. Forest Service Law Enforcement and Investigations unit, would not say Thursday whether it planned to press for charges against the hunter.

The Rim Fire started several hundred feet above the Clavey River, below the promontory Jawbone Ridge, in a section of forest that is rarely visited, according to forest service officials. There are no public roads or designated trails in the area.

The closest access is from a gated forest-service road that leads partially up Jawbone Ridge but is still more than mile away from where the fire started. The road is closed to public vehicle traffic.

Deer hunting, though, is popular in the national forest this time of year, and officials had speculated that hunters might have hiked into the Clavey River canyon. The archery season goes through Sunday, and rifle hunting begins Sept. 21.

Hunters are not required to register their visits.

A dry spring and recent hot weather made the canyon particularly prone to fire, and even though parts of the area had burned before, a great deal of brush and pine remained to fuel the blaze.

Pot farm unlikely

Rumors had circulated that illicit marijuana growers were responsible for starting the blaze, but forest service officials said pot gardens were unlikely in this part of the forest because of its steep terrain and lack of water for irrigation.

Investigators also had ruled out lightning because no strikes were reported at the time the fire started.

William Stewart, a forestry specialist at UC Berkeley, said most wildfires in California, especially in low-elevation foothills, are caused by people.

"Unlike the Rocky Mountains, the Sierras don't actually get that much lightning in the summertime, and (for a fire) we need lightning without rain following it," Stewart said. "We just don't have a lot of dry lightning."

Stewart said he was surprised to hear that a hunter was responsible for the blaze. Most wildfires, he said, are started by people who are less knowledgeable about the outdoors.

Over the past few days, firefighters have made substantial progress corralling the Rim Fire. Full containment is expected by Sept. 20, but it's expected to smolder well into the fall rainy season.

Highway 120 through much of Yosemite National Park remains closed, though the heavily visited Yosemite Valley can still be reached on Highway 140 from Merced and Highway 41 from Fresno.