Soboba Hot Springs, Calif. ? The mother of one of the four firefighters who died battling a wildfire that authorities blamed on arsonists urged those who set it to turn themselves in Saturday.

“I firmly believe you didn’t believe that things were going to turn out the way they did, but they did,” said Bonnie McKay, whose son Jason, 27, died Thursday. “Don’t let the remorse eat you alive. Come forward. … I, for one, will try not to judge you. There is only one who can judge you.”

Meanwhile, firefighters took advantage of calm weather and dissipating Santa Ana winds, making headway against the 62-square-mile conflagration by dumping water and retardant on flames using a fleet of helicopters and airplanes, including a DC-10 jumbo jet.

Still, forestry officials worried about the fire spreading in one area.

Scott McLean, spokesman for the California Department of Forestry, said the southeastern flank was threatening to spread to Black Mountain, a forested area even steeper than where it is burning now.

“If it goes there, the fire is going to hell in a handbasket,” said McLean from the command post in Beaumont, 90 miles east of Los Angeles.

The 39,900-acre blaze was 40 percent contained, two days after blowtorch gusts overran a U.S. Forest Service crew, killing four of its members and leaving a fifth clinging to life with burns over most of his body.

Crystal McKay, center, sister of firefighter Jason McKay, reacts during news conference with the McKay family at the Apple Valley Fire District Station One on Saturday in Apple Valley, Calif. Firefighter Jason McKay was one of four Forest Service firefighters killed Thursday when they where overrun by a wind-driven wildfire.

Firefighter Pablo Cerda, 23, was in critical condition Saturday at Arrowhead Regional Medical Center after surgery Friday to remove damaged skin.

Investigators combed the area Saturday, looking for clues on how the fire engulfed the men so quickly. They interviewed a handful of firefighters who were nearby when it happened, Al Matecko, spokesman for a national investigating team.

A reward for information leading to the arsonist soared to $500,000 Friday, as $100,000 posted by Riverside County quickly multiplied with matching offers from the state, neighboring San Bernardino County, Rancho Mirage resident Tim Blixseth and the Morongo Band of Mission Indians.

Investigators were looking into whether the wildfire was related to other blazes in recent months, including a canyon fire last weekend, though a sheriff’s spokesman said there was no immediate indication of a serial arsonist.

Residents said they saw two young men leaving the area where the fire broke out early Thursday west of the San Jacinto Mountains.