When a small-town fire chief suggested last month that the Rim Fire was sparked by marijuana growers, the theory was seized upon as a leading explanation of the blaze.

The Aug. 23 comments of Twain Harte Fire Chief Todd McNeal were circulated on YouTube and hyped on Twitter and Facebook, and his speculation fueled news reports nationwide.

It turns out they were probably wrong.

The U.S. Forest Service, which is leading the investigation of the giant fire in and around Yosemite National Park, says the pot-grower explanation is highly unlikely - because the fire's starting point in the Clavey River canyon in the Stanislaus National Forest is hard to get to, too steep for growing plants and doesn't have water for irrigation.

"We do have problems with marijuana grow sites, but in this area it's very difficult terrain," said Jerry Snyder, a Forest service spokesman who has worked in the area for 11 years.

Snyder and others involved in a multiagency effort to root out the cause of the Rim Fire, which on Wednesday had grown to 370 square miles, have yet to provide their own definitive answers to the mystery of what triggered the blaze. And that's part of the problem.

In the absence of an explanation, rumors have spread.

McNeal told residents of the foothill community of Twain Harte, one of the towns advised to evacuate because of wildfire danger, that the fire was likely human-caused because there was no lightning when it started: It's "highly suspected it might have been some sort of illicit (marijuana) grove," he said at a public meeting.

McNeal could not be reached for comment Wednesday.

The U.S. Forest Service has been trying to squelch the pot-farm theory for nearly two weeks - and others, too.

The agency says there was no prescribed burning by forest managers in the area, which some have speculated could have ignited the blaze. The agency also says federal drug agents were not working in the forest, countering rumors that the federal government might have accidentally lit the fire.

There are plausible theories still on the table, fire officials said - but they declined to comment on which ones they're investigating.

The area where the fire started, about 2 miles northeast of the community of Buck Meadows, outside Yosemite, is difficult to access, but it's possible that someone was passing through and either purposely or inadvertently started the trouble. An extreme backpacker or a hunter in search of deer could have left behind a campfire or failed to extinguish a cigarette.

The Clavey River, which empties into the nearby Tuolumne River, draws kayakers, too - but water levels have been low this summer, so it's unlikely that anyone was traveling by river, officials said.

Lightning was not reported in the area when the Rim Fire began Aug. 17. That doesn't leave much room for a natural cause, though a rockslide remains a possibility, officials said.

"Have you ever banged two rocks together and gotten a spark?" said Snyder.

Daniel Berlant, spokesman for the California Department of Forestry and Fire Prevention, said the area was particularly fire-prone because of the hot weather and long-parched landscape.

"Let's be realistic. When you have dry conditions, it doesn't take much to start a fire," he said.

Cal Fire is assisting with the investigation, as are the Sonora Police Department and Tuolumne County district attorney's office. The official cause of the fire could take months to determine.

Ignition is believed to have been several hundred feet above the Clavey River, where flames then proceeded uphill over the steep Jawbone Ridge and deeper into the national forest.

Firefighters reported Wednesday that they continue to make progress on what's become California's fourth-largest wildfire on record, and with the advances, hundreds have been released from the fire lines. That puts the total force at 4,100 firefighters, down from more than 5,000 just days ago.

Evacuation advisories have been lifted on the fire's northern edge in the rural communities along Highway 108 above Sonora. Farther south, near Groveland, residents remain under an evacuation order.

The fire is 80 percent contained, and full containment is expected by Sept. 20.