The fire is the largest in the history of Los Angeles county
A huge wildfire which has devastated a large area north of Los Angeles was started deliberately, say the authorities in California.
A homicide investigation has been launched as two fire fighters were killed tackling the blaze, known as the Station Fire.
The fire covers nearly 150,000 acres - the largest in the county's history.
Dozens of homes have been destroyed and thousands of people evacuated but the fire is now 38% contained, say reports.
"After a forensic examination at the point of origin, arson investigators have concluded that the Station Fire was the result of an act of arson," said US Forest Service Commander Rita Wears.
"The Los Angeles County Sheriff's Department homicide bureau has initiated a homicide investigation due to the tragic death of the two Los Angeles County firefighters as a result of this fire."
No further details have been given about the evidence for arson or whether any suspects have been identified.
The LA Times quoted Los Angeles County Sheriff Lee Baca as saying making details public could damage attempts to find and prosecute suspected arsonists.
Officials says Station Fire could burn until mid-September
The two firefighters, Ted Hall and Arnie Quinones, were killed when their fire engine fell down a mountainside at the weekend.
US Forest Service Commander Mike Dietrich said 4,800 firefighters were still working to contain the Station Fire.
Mr Dietrich said "excellent progress" had been made but there was still "a little way to go".
"We're changing the pace and treating this as a marathon. If it were a 26-mile race, we'd only be at mile six," he said.
A total of 64 homes have been destroyed and 147,418 acres of land burned. The fire is not expected to be fully contained until mid-September.
Mr Baca told the LA Times that "everyone should be angry" about the situation, saying arson was "one of the most unacceptable crimes".
"This fire destroyed half of one of the best natural resources, the Angeles National Forest. It sent thousands from their homes and above all else cost the lives of two heroic firefighters," said Mr Baca.
The governor of California, Arnold Schwarzenegger, toured affected areas on Thursday and said it was "horrible" for those who had lost their homes and belongings.
"Even though we are still battling those fires, we are now trying to help get people's lives rebuilt," he said.
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