The investigation into a deadly Los Angeles wildfire became a homicide probe on Thursday as authorities revealed that the mammoth blaze burning near the city was caused by arson.
As weary crews tightened their grip on the monster inferno, which has been blamed for the deaths of two firefighters since it erupted on August 26, officials confirmed the blaze was set deliberately.
"Arson investigators have concluded that the fire was the result of an act of arson," said US Forest Service official Rita Wears.
"The Los Angeles County Sheriff's Department homicide bureau has initiated a homicide investigation due to the tragic deaths of the two Los Angeles County firefighters as the result of this fire."
The two firefighters -- Ted Hall, 47, and Arnie Quinones, 35 -- were killed when their truck plunged down a mountainside as they battled flames in a remote location on Sunday. Quinones's wife is eight-and-a-half months pregnant with the couple's first child, according to reports.
The fire -- the largest in Los Angeles County's history -- has burned more than 147,000 acres (59,400 hectares) and destroyed 64 homes in just eight days.
However higher humidity and cooler temperatures across the region allowed firefighters to increase containment of the fire to 38 percent, officials said earlier on Thursday.
Fire commander Mike Dietrich said "great progress" had been made overnight, but noted that flames spreading into remote mountain regions had forced the evacuation of 25 residents from 11 homes.
A total of 12,000 homes and 500 commercial buildings are officially listed as threatened by the fire, but most of the estimated 10,000 people affected by mandatory evacuation orders have been allowed to return to their homes.
The US Forest Service said the fire had moved into areas of the Angeles National Forest that have no previous history of wildfires.
Officials said 4,735 firefighters are battling the blaze, backed up by 11 air tankers, nine helitankers, 11 helicopters, 433 fire engines and 60 bulldozers used to carve out firebreaks in the rugged terrain.
Dietrich said the threat to Mount Wilson -- home to a historic observatory and communications towers used by several local television, radio and law enforcement agencies -- had diminished.
California is frequently hit by wildfires due to its dry climate, winds and recent housing booms that have seen home construction spread rapidly into rural and densely forested areas.
But Los Angeles Fire Department Captain Jerry Meehan meanwhile this fire was the largest in the area's history. "This is the biggest ever," Meehan said. "This is the hottest-burning, most damaging I've ever seen."
In 2007, the state suffered some of the worst devastation from wildfires in its history as the disaster claimed eight lives, gutted 2,000 homes, displaced 640,000 people and caused a billion dollars of damage.