Luis Castillo, a firefighter with the California Department of Forestry, battles the Esperanza fire Friday. The blaze killed four U.S. Forest Service firefighters Thursday and has scorched more than 39,900 acres in the mountains west of Palm Springs, Calif.

Beaumont, Calif. ? As flames roared through the canyons of Southern California, authorities increased the reward to $500,000 Friday for the arrest of the arsonist responsible for the deaths of four firefighters in the nation’s worst such tragedy in five years.

A fifth firefighter lay in grave condition with burns over 90 percent of his body.

Firefighters labored through thick smoke as curtains of wind-whipped orange flame pushed through uninhabited brushland about 90 miles east of Los Angeles. The blaze that broke out Thursday destroyed about 10 structures, forced 700 people to flee and flushed coyotes and other wildlife into the open.

Investigators have not said how they know the 62-square-mile 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 1 a.m. 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 2,070 firefighters worked to corral the advancing flames and a DC-10 jet capable of dropping 12,000-gallon loads of retardant joined a fleet of firefighting helicopters and airtankers. Winds gusted to 45 mph but kept the flames moving through undeveloped land, away from homes in Riverside County.

The blaze was 25 percent contained, officials said.

The U.S. Forest Service firefighters were killed Thursday when the wind blew a wall of flames down on them in the hills near Palm Springs as they tried to protect a home.

Killed were engine Capt. Mark Loutzenhiser, 44; engine operator Jess McLean, 27; assistant engine operator Jason McKay, 27; and firefighter Daniel Hoover-Najera, 20. Another crew member, Pablo Cerda, 23, was in grave condition.