Jerry Snyder of the U.S. Forest Service said Wednesday that it's unlikely that the fire started at an illegal marijuana grow site. Snyder said the steep and inaccessible canyon where the Rim Fire started Aug. 17 in the Stanislaus National Forest doesn't have the kind of water source that growers look for when they set up remote growing sites.
"I've heard many rumors, and that's one of them," Snyder said. "This isn't the kind of terrain growers look for. They want an isolated area, but they want access. This doesn't have it."
Snyder also knocked down lightning as a possible cause, adding it could take months to pin point a cause for the fire that has consumed more than 370 square miles of Sierra Nevada forests.
"They'll be able to tell whether there was an illegal campfire in there," he said. "Another thing to consider is that this area is very steep, and if there was a rockslide two rocks hitting together could make a spark to ignite dry brush."
The blaze is currently 80 percent contained. Full containment is not expected until Sept. 30. The far-off date is because the portion of the fire burning in Yosemite National Park is headed toward granite outcroppings that will act as a natural firebreak but won't be classified as technical containment.
Letting the granite formations help will allow crews to focus some efforts inside the fire's footprint. Snyder said they have begun to cut breaks and start backfires in an effort to save grazing land, wildlife habitat and historic buildings left over from early timber camps.
Officials said 111 structures, including 11 homes, have been destroyed. More than 4,300 firefighters are still battling the blaze.
Although no cause has been announced, one local fire chief speculated the fire might have ignited in an illegal marijuana grow. His remarks posted on YouTube prompted Snyder to shoot down the rumor.
The Associated Press contributed to this report.