The Rim Fire has scorched hundreds of square miles near Yosemite National Park.
The massive California wildfire that scorched Yosemite National Park and posed a threat to San Francisco's water and electricity supplies was started when a hunter's illegal camp fire got out of control, according to the U.S. Forest Service.
The Forest Service announced the alleged cause of the blaze, which ate up more than 237,000 acres -- or 370 square miles -- on Thursday. The fire is 80 percent contained.
Officials have estimated that $70 million has been spent fighting the blaze.
The fire burned through brush, conifer and pine trees, growing into the fourth largest in the state's history since it began on Aug. 17. Plumes of smoke from the fire prompted air-quality warnings a hundred miles away and chased off would-be day campers in Yosemite.
The fire scorched 370 square miles, and authorities say an illegal fire started it all. NBC's Brian Williams reports.
Firefighters expect that it will take about two more weeks to fully contain the fire, according to an incident report.
"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 a release on Thursday. "No arrests have been made at this time and the hunter's name is being withheld pending further investigation."
A fire chief in Tuolomne County had earlier speculated that an illegal marijuana growing operation might have been the cause of the blaze. Todd McNeal, fire chief of a town west of Yosemite, said on Aug. 23 that there was reason to "highly suspect" that a marijuana grove may have helped spark the fire.
Others have said that better funding for tree-thinning projects could have slowed the spread of the wildfire and kept it from getting so large, according to Reuters. John Buckley, executive director of the Central Sierra Environmental Resource Center, said that such efforts would have "inarguably made the Rim Fire far easier to contain, far less expensive and possibly not even a major disaster," according to the news service.
Elisha Fieldstadt of NBC News contributed to this report.