Officials said forensic evidence at the fire's point of origin revealed that the wildfire - among the largest in Southern California's history - was an act of arson. Whitmore declined to elaborate on the evidence due to the ongoing investigation.
The two firefighters died Sunday when their truck careened off a steep mountain road.
Los Angeles County Fire Deputy Chief Mike Bryant said he was glad investigators were making progress, but that the arson finding "doesn't mend my broken heart," he said.
"We got to put this fire out so no one else gets hurt," he added.
Earlier Thursday, Gov. Arnold Schwarzenegger toured a fire-ravaged community where the fire left dozens of homes in ruins, encountering piles of twisted metal and rubble as firefighters began to bring the blaze under greater control.
The blaze was 38 percent contained Thursday, up from 28 percent the previous day.
Schwarzenegger talked to residents about their losses and later thanked firefighters for all of their work in putting out the flames. At one point during the tour, the former bodybuilder picked up a 30-pound barbell located amid the wreckage.
"Even though we are still battling those fires, we are now trying to help get people's lives rebuilt," Schwarzenegger said. "When you see this kind of devastation, it's horrible to lose your home, your personal belongings."
Despite the overall progress against the fire, firefighters dealt with a flare-up overnight in a remote canyon as strong downslope winds "just kind of blew the fire up," said U.S. Forest Service official John Huschke. Twenty-five people in 11 homes were evacuated in the canyon area.
The wildfire, now in its eighth day, has destroyed 64 homes, burned three people and left two firefighters dead. During the night, a firefighter injured his leg when he fell 20 foot from a cliff and was taken to a hospital by a medical helicopter, officials said. He was in stable condition.
Full containment was expected Sept. 15, meaning fire officials expect that they will have the blaze completely surrounded by then.
Firefighters have been conducting an aerial assault on the fire to complement efforts on the ground. Helicopters have doused the fire with 1.7 million gallons of water - enough to fill about three Olympic-sized swimming pools - while airplanes have dropped 670,000 gallons of retardant on the fire.
"We're changing the pace and treating this as a marathon," U.S. Forest Service incident commander Mike Dietrich said. "If it were a 26-mile race, we'd only be at mile six."
There were growing signs that Los Angeles was looking to move beyond the fire.
The UCLA football team earlier in the week feared it might have to postpone its home opener on Saturday because the fire is so close to the Rose Bowl, its home stadium. But the school said Thursday that the game will be played as scheduled.
Schwarzenegger got an earful from some residents as he toured the community of Vogel Flats in Big Tujunga Canyon, where most of the 40 homes were leveled by the blaze.
Bert Voorhees, 53, who lost his 800 square-foot home, wondered why firefighters didn't have aircraft or strike crews available before the fire raced into the canyon over the weekend and wiped out the mountain community.
"I just know a terrible mistake was made in this canyon," said Voorhees, a civil rights lawyer. "It's much bigger than this canyon. The fact that it cost two guys (firefighters) their lives, it's like bigger than any of this."
Voorhees suggested that fire officials bowed to political pressure and opted to protect richer neighborhoods to show off its aerial assault instead of snuffing out the fire when it was in its infancy.
Fire officials denied they were influenced by legislators where to put firefighters and equipment. They said they were willing to meet with residents about what happened.
"The distance of this fire in relation to this canyon, necessitated putting resources where the immediate threat was," deputy incident commander Carlton Joseph said. "This thing moved in hard and fast. Firefighters told me they have never seen a fire move that fast."
The search for what sparked the blaze intensified as U.S. Forest Service investigators gathered along a road in a blackened forest to hunt for clues near where the fire started. They shook soil in a can and planted red, blue and yellow flags to mark evidence beneath a partially burned oak tree at the bottom of a ravine.
Associated Press writers Thomas Watkins, Jacob Adelman and Daisy Nguyen in Los Angeles contributed to this report.