Four firefighters have died while trying to tackle a big brush fire near Palm Springs in California, some 200km east of Los Angeles.
Officials believe the fire was started deliberately and hundreds of people have had to flee their homes.
Local police spokesperson Neil Lingle said there would be a reward of $100,000 for anyone who helped catch the arsonist.
California Governor Arnold Schwarzenegger said it was a 'very sad day' for the state.
The fire added to the record destruction of US forest land this year by wildfires: more area has been charred across the country than any year since authorities began keeping statistics in 1960.
In just ten months of 2006, more than 9.6 million acres have been consumed, well past last year's 12-month record of 8.7 million acres.
California's fires have been exacerbated this year by a deep heat wave.
In September, a fire ravaged 120,000 acres of mountainous forest northwest of Los Angeles. Another fire in July swept through the scrub desert and mountains north of Palm Springs.
