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 63-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 that is difficult to access.

“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 40,450-acre blaze was 60 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.

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.

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.

A law enforcement official described the investigation as being in its “infancy.”