Four firefighters have died in the blaze, America’s worst such tragedy in five years, while a fifth firefighter lies in grave condition with burns over 90 percent of his body.
The deaths and injuries occurred when the wind blew a wall of flames down on them in the hills near Palm Springs as they tried to protect a home.
Firefighters are labouring through thick smoke as curtains of wind-whipped orange flame pushed through uninhabited brushland about 145 kilometres east of Los Angeles.
The blaze destroyed about 10 structures, including homes, forced 700 people to flee, and flushed coyotes and other wildlife into the open, yesterday.
Investigators have not said how they know the nearly 100-square-kilometre blaze was arson, how it was set, or why but they said those responsible could face murder charges.
Two young men were seen leaving the area where the fire broke out about 1am on Thursday.
Also, investigators said they were looking into whether the wildfire was related to other blazes in recent months, including a canyon fire last weekend.
More than 1,700 firefighters worked to corral the advancing flames.
Mop-up work was under way back in the mountain communities where the flames have already swept through. About 400 people remained evacuated.
Californian Governor Arnold Schwarzenegger added US$100,000 ($A131,155) in state money to a reward already posted by Riverside County for information leading to the arsonist.
San Bernardino County and the Morongo Band of Mission Indians, which has a casino, added another US$200,000 and Rancho Mirage resident Tim Blixseth also donated US$100,000 bringing the reward’s total to US$500,000 for the arsonist’s capture.