The fire was reported at about 2 p.m. near a Bureau of Land Management campground along the highway, said Cal Fire spokesman Kevin Colburn.
The clouds of smoke rising from the fire could be seen from around the south county.
By 7:30 p.m., the blaze had grown to 350 acres in very steep, rugged terrain, with the fire burning through grass, brush, oak and some pine, Colburn said.
At that time Colburn said the blaze was 40-percent contained, with between 250 and 350 firefighters on scene trying to bring it under control.
Two outbuildings and a residence were destroyed, said Colburn, and one firefighter suffered a heat-related injury.
Colburn, who is a part of the team that's still investigating the case, said the fire was caused by a vehicle, but he didn't reveal specifically how the fire started.
Among the personnel on scene were Cal Fire firefighters and equipment from stations in Lake County, said Colburn.
The Rumsey Rancheria Fire Department also sent firefighters and equipment to assist, he said.
“Crews are going to be out all night working on getting the other 60 percent contained,” said Colburn.
The California Highway Patrol reported that Highway 16 at Highway 20 will be closed until sometime on Tuesday because of the fire.
John Jensen contributed to this report.
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