Official blames Rim fire on marijuana growers [UPDATE] Firefighters from Sacramento Metro burn around an old cabin in Ackerson Meadow near Yosemite National Park, California on August 28, 2013. The enormous Rim Fire became the sixth-largest wildfire in recorded California history at 192,500 acres. UPI/Al Golub 3 (UPI) -- The Rim fire, burning in northern California, was caused by humans and possibly started by an illegal marijuana-growing operation, a fire official said. 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. The central Sierra Nevada blaze is the nation's largest active forest fire and California's fourth-largest in history, the agency 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. 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.