Deadly Yosemite fire’s cause: vehicle sparks

A Grumman S2 Tracker air tanker, like the one pictured here, crashed while fighting the Dog Rock Fire in Yosemite National Park on Tuesday, Oct. 7, 2014. A Grumman S2 Tracker air tanker, like the one pictured here, crashed while fighting the Dog Rock Fire in Yosemite National Park on Tuesday, Oct. 7, 2014. Photo: Williams, Kale, Cal Fire Photo: Williams, Kale, Cal Fire Image 1 of / 1 Caption Close Deadly Yosemite fire’s cause: vehicle sparks 1 / 1 Back to Gallery

The Yosemite wildfire that claimed the life of a San Jose air tanker pilot, who crashed as he battled the blaze, was likely ignited by sparks from a vehicle, authorities said Thursday.

The exact source of ignition remains unclear, but investigators believe metal fragments such as a dragging trailer chain, or brake parts that heated up, kindled a roadside blaze along Highway 140 on Tuesday, which exploded into the hills near the park’s entrance near El Portal.

The Dog Rock Fire grew to 245 acres by Thursday afternoon, and remained just 10 percent contained. The Highway 140 entrance to Yosemite National Park remained closed, and power was temporarily cut off to Yosemite Valley.

Soon after the fire broke out, a plane contracted by the California Department of Forestry and Fire Protection to drop fire retardant crashed into a canyon near where the wildfire was burning, killing 62-year-old Geoffrey “Craig” Hunt.

The crash prompted Cal Fire to ground its firefighting S2 Tracker fleet statewide as a safety precaution. Aircraft from the National Park Service began flying missions over the Dog Rock Fire.

Firefighters on Thursday succeeded in keeping the wildfire from crossing Highway 140, but the blaze continued to spread out of control high above the road. Steep, inaccessible terrain complicated the firefighting effort.

Kurtis Alexander is a San Francisco Chronicle staff writer. E-mail: kalexander@sfchronicle.com

Twitter: @kurtisalexander