There was no evidence of an illegal marijuana grow near the spot where a raging wildfire started near Yosemite National Park, a federal forestry official said Wednesday.

Investigators have ruled out the illicit activity as a potential cause, ending speculation by a local fire chief that the gardens that plague federal land could be to blame.

Jerry Snyder of the U.S. Forest Service said that the steep and inaccessible canyon where the Rim Fire started Aug. 17 in the Stanislaus National Forest doesn't have a water source that growers look for when they set up remote gardens.

"The lead investigator says there's no evidence of any type of grow in the area where the fire started,'' Snyder said.

Snyder also said lightning isn't to blame. It could take months for investigators to determine what ignited the blaze 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 fire is 80 percent contained, and crews don't expect full containment before Sept. 20. 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 geological formations help will allow firefighters 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.

"We don't want the entire interior to be burned too,'' he said.

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.