Chief Todd McNeal of the local Twain Harte Fire and Rescue company told a community meeting on Aug. 23 the massive fire, which has burned nearly 370 square miles or nearly 240,000 acres since it began Aug. 17, started in a section of the Stanislaus National forest inaccessible by foot or vehicle. He added it was "highly suspected" an illegal marijuana-growing operation in the forest sparked the blaze.
The U.S. Forestry Service and California Department of Forestry and fire Protection officials only say the cause of the fire is under investigation, the Los Angeles Times reported.
The fire, now burning in and around California's Yosemite National Park was 70 percent contained, up from 45 percent a day earlier. Full containment is still not expected until Sept. 20, the California Department of Forestry and Fire Protection said.
The central Sierra Nevada blaze is the nation's largest active forest fire and California's fourth-largest in history, the agency said.
Some 5,000 firefighters battling the 17-day-old blaze got some Labor Day relief from cooler temperatures, higher humidity and cloud cover, U.S. Forest Service spokesman Mark Healey said.
Winds gusting to 22 mph also switched directions and were expected to blow northward Tuesday, he said.
"That should give the firefighters working the southern flank a break, but it'll challenge the firefighters on the north, east and west flanks," Healey told the San Francisco Chronicle.
"Generally speaking, we expect to see the fire continuing to clean up interior pockets of unburned fuel," he said.
State Route 120, which runs through Yosemite, remained closed from Crane Flat to Yosemite Creek as was the Yosemite park entrance from the highway. Other entrances were open.
Water quality remained high at the park's 117 billion-gallon Hetch Hetchy Reservoir, the source of unfiltered water for 2.6 million Bay Area residents and businesses from San Francisco south to Silicon Valley, officials said.
San Francisco's Public Utilities Commission has backup reservoirs on standby if ash falling into the Hetch Hetchy from the forest fire compromises water quality, officials said.
Utility officials stopped using two hydroelectric plants in the area because of the fire, but that has not affected power delivery to San Francisco customers, the Chronicle said.
No structures have burned since last week, when officials reported 111 were destroyed -- mostly small cabins and outbuildings -- but 11 homes were among those destroyed. About 4,500 structures remain threatened.