Fire spokesman John Huschke said early Friday that "it's pretty quiet" on the fire lines as hand crews and bulldozers clear a containment line around fire's perimeter. The blaze is 38 percent surrounded.
An unidentified source said that evidence along Angeles Crest Highway includes incendiary material.
Los Angeles County sheriff's officials started the investigation Thursday, five days after two firefighters died when their truck plunged 800 feet down a steep mountain road.
Los Angeles County Deputy Fire Chief Mike Bryant said he was glad investigators were making progress in the probe, but "it doesn't mend my broken heart."
"Those were two great men that died," he said. "We've got to put this fire out so no one else gets hurt."
County fire Capt. Ted Hall and Specialist Arnaldo Quinones supervised a fire crew, made up mostly of prison inmates, at a forest campsite. They died Sunday on Mount Gleason while trying to find an escape route for the crew after flames overran the camp.
Hand crews and water-dropping helicopters had almost contained the fire's western flank in the rugged canyons near Pacoima, but 65 miles of fire line have yet to be cut, U.S. Forest Service Incident Commander Mike Dietrich said.
A historic observatory and TV, radio and other antennas on Mount Wilson, which at one point was dangerously close to the flames, are "looking pretty darn good," he said, but the fire is pushing east into the wilderness and down toward foothill cities of Monrovia, Sierra Madre and Pasadena.
Even in a landscape blackened by wildfire, clues abound for investigators following the path of a blaze and trying to find out how it started. Investigators start at the place where firefighters were first called and work backward.
Jeff Tunnell, a wildfire investigator for the Bureau of Land Management, said even in charred terrain, investigators can detect important signs in the soot.
"Fire creates evidence as well as destroys it," said Tunnell, a veteran of 50 wildfires who is based in Ukiah. "We can follow fire progression back to the point at which it started."
Clues can come from burned trees and grasses, where the amount of burned foliage can show the direction and speed a fire was moving. Investigators search for the remains of whatever started the fire: a charred match or cigarette butt, a piece of metal from a car or part of a power cable. If no such object is found, they often conclude that a fire was "hot set," meaning it was started by a person holding a lighter to the brush.
"That's what you are going to assume, because there's no other competent ignition source," he said.
Most wildfires are caused by human activity. Even a fire caused by a singed squirrel tumbling from an electrical transformer is designated as human-caused, because humans put the electric box there, Tunnell said. Other wildfire causes are lightning and volcanoes.
At the time the current fire broke out, Forest Service officials said there was no lightning and no power lines nearby.
Three years ago, arson investigators probing the cause of a wildfire in the San Jacinto Mountains that killed five firefighters discovered evidence of different types of incendiary devices at several fires. They recovered everything from simple paper matches to more elaborate devices made up of wooden matches grouped around a cigarette and secured with duct tape or a rubber band.
The evidence was enough to build a first-degree murder case against mechanic Raymond Lee Oyler. In March, the evidence was used to convict him and send him to death row.