Rim Fire: Hunter's illegal fire set off huge wildfire near Yosemite, USFS says (Map, Fire Tracker)

A firefighter uses a hose to douse the flames of the Rim Fire on Saturday, Aug. 31, 2013 near Groveland, California.
A firefighter uses a hose to douse the flames of the Rim Fire on Saturday, Aug. 31, 2013 near Groveland, California.
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The U.S. Forest Service said Thursday that a hunter – who let an illegal campfire get out of control – caused the massive Rim Fire near the western edge of Yosemite National Park.  No arrests have been made and investigators are trying to find out more.

The forest service released a statement announcing the cause of the wildfire and has so far declined a request for an interview.

Determining the cause of these kinds of wildfires is challenging.    

Bill Stewart, Forestry Cooperative Extension Specialist with the College of Natural Resources at UC Berkeley, has worked closely with people who have investigated wildfires and says timing is crucial.  

“If you can’t find the evidence in the first few weeks, you pretty much close it up," Stewart said. "Unless there’s evidence there was some vehicle or human activity in that region.”  

Rim Fire investigators have determined that the nearly three-week old wildfire began when a hunter allowed an illegal fire to spread in a remote area of the Stanislaus National Forest - near an area known as Jawbone Ridge, where there is thick brush.  

Stewart adds that discovering how a wildfire starts is a huge step forward because, many times, officials don’t get that far.

“Usually the largest cause is the unknown category," Stewart said. "All the evidence is burnt up, no one was there and if they were there they didn’t tell anybody.”  

Stewart says investigators likely laid out a grid system to help them find the cause while other firefighting crews worked to contain the fire. Officials had previously ruled out that lightning was to blame.

The Rim Fire began Aug. 17th and has cost about $77 million to fight. 

It has burned 237,341 acres and is 80 percent contained.  The wildfire is the fourth largest in California history and has destroyed 111 buildings - including 11 residences. 

At its peak, the Rim Fire threatened San Francisco's water supply and Yosemite's ancient sequoias. The fire could be seen from space and helicopter images were stunning.

This story has been updated.