Hunter's campfire sparked California blaze that scorched Yosemite SAN FRANCISCO (Reuters) - A monster blaze that has blackened an area larger than Dallas in the rugged northern California forests in and around Yosemite National Park started from an illegal campfire a hunter built and let grow out of control, fire managers said on Thursday. The precise origins of the so-called Rim Fire, which erupted nearly three weeks ago in the Stanislaus National Forest west of Yosemite, remained under investigation, and no arrests have been made, the U.S. Forest Service said. But the campfire that sparked the conflagration, now the fourth-largest California blaze on record, was built in defiance of Forest Service restrictions imposed at the time due to dangerously hot, dry conditions in the area, officials said. Whether the hunter deemed to be the culprit will face criminal charges depends on a variety of factors, including the degree of carelessness or neglect determined by investigators, said Forest Service spokesman Ray Mooney. Beyond saying the fire began "when a hunter allowed an illegal fire to escape," authorities declined to comment on evidence linking the hunter to the blaze. The individual has not been publicly identified. The Rim Fire, named for a popular local lookout spot called Rim of the World, has charred more than 237,000 acres, or 370 square miles (958 square kilometers), of timber and chaparral since it erupted August 17 in an area known as Jawbone Ridge in a remote corner of the national forest. Firefighting costs there have run $81 million to date, with some 5,000 personnel assigned to battle the blaze at its peak. With a footprint bigger than the land mass of Dallas, Texas, the blaze ranks as the largest of dozens of wildfires that have raged across several states in the drought-parched West this year, straining U.S. firefighting resources. Dozens of homes and cabins near Yosemite have been destroyed and nearly 2,500 dwellings in the region were still threatened by the blaze on Thursday. But no serious injuries have been reported, and most evacuation orders have been lifted as firefighters tightened their grip on the flames. As of Thursday, ground crews armed with hand tools and chainsaws, and backed by bulldozers, water-dropping helicopters and airplane tankers with payloads of chemical retardant, had cut containment lines around 80 percent of the fire's perimeter. But fire managers say they expect the blaze to continue burning for several more weeks, and warned that a resumption of gusty winds "could challenge containment lines." Less than a third of the total burned acreage lies inside Yosemite, and firefighters there have succeeded in limiting most of the damage to wilderness and back-country areas in the park's remote northwestern corner. Several campgrounds, numerous trails and one of the four main entrances to the park have been closed by the fire. But the most popular portions have remained open to the public, including the scenic Yosemite Valley area famed for its towering granite rock formations, waterfalls and meadows. Nevertheless, park officials say droves of visitors who typically crowd Yosemite in late summer have noticeably diminished. And Yosemite-area businesses say the region's tourist economy has been hit hard a year after an outbreak of hantavirus in the park frightened away many park visitors. Word that investigators had traced the fire's origins to a hunter's campfire came a day after authorities ruled out the possibility that the Rim Fire somehow began from an illegal marijuana cultivation site in the national forest. Speculation that a pot farm might be to blame grew out of videotaped comments made by a local fire department chief during an August 23 community meeting, but fire officials again denied any such connection to the blaze, or to the hunter in question. "There is no indication the hunter was involved with illegal marijuana cultivation on public lands and no marijuana cultivation sites were located near the origin of the fire," the Forest Service said in its statement. Writing by Steve Gorman; Editing by Ken Wills