A fifth U.S. Forest Service firefighter died Tuesday of burns suffered when an engine crew was overrun by a Southern California wildfire, while investigators searched for the arsonist who set the blaze that burned 63 square miles.

Pablo Cerda, 23, of Fountain Valley, died at 5:08 p.m. at Arrowhead Regional Medical Center in Colton, Jeanne Wade Evans, the San Bernardino National Forest supervisor, said at a news conference outside the hospital. A group of Forest Service firefighters with tears in their eyes stood behind her.

"I felt the faith and hope for Pablo's recovery and actually felt a miracle might be possible," she said. "Today more sadness is added to our almost unbearable grief."

Cerda was burned over 90 percent of his body Thursday as he and the crew of Engine 57 tried to protect a home from wind-driven flames in the San Jacinto Mountains about 90 miles east of Los Angeles. Three other crew members died at the scene and Cerda's captain died soon after at a hospital.

Cerda underwent extensive removal of damaged skin in an operation Friday but was given a poor prognosis.

Dr. Dev Gnanadev said Cerda's family was given the option of returning him to the operating room but "they decided to let Pablo go." He was taken off life support.

Authorities said the fire was deliberately set early Thursday at the base of a slope in Cabazon, west of Palm Springs, as fierce winds blew through the region. Before firefighters contained it Monday, the blaze scorched 40,200 acres and destroyed 34 homes and 20 outbuildings. A portion of a highway in the fire area remained closed indefinitely for repair.

Firefighting costs reached $9.9 million, the California Department of Forestry said.

Residents said they saw two young men leaving the area where the fire began.

On Tuesday, authorities interviewed previously convicted arsonists who live in the area, said James Crowell, an agent with the Bureau of Alcohol, Tobacco, Firearms and Explosives who is leading the investigation. In California, convicted arsonists must register with the county and provide their address.

A day earlier, two people were brought in for questioning and released. No arrests have been made.

At the crime scene, investigators had planted blue, red and yellow flags attached to wire stakes in the ground to mark the location of possible evidence. Part of the hillside was marked by a grid made of pegs and string.

Dozens of investigators from the ATF, FBI and state and local agencies were sifting through hundreds of leads, ATF spokeswoman Susan Raichel said. The reward for information leading to an arrest had grown to $550,000.

A memorial service for the dead firefighters was set for Sunday.

The five deaths were the most in a single incident while battling a wildfire since 14 were killed in July 1994 during a blaze near Glenwood Springs, Colo., according to National Interagency Fire Center statistics.

The other victims were engine captain Mark Loutzenhiser, 43, of Idyllwild; Jason McKay, 27, of Apple Valley; Jess McLean, 27, of Beaumont; and Daniel Hoover-Najera, 20, of San Jacinto.

"All of us from the forest and all those from the fire service deeply mourn the loss of these brave men. In my mind there's no greater calling than to help those in need," Evans said.