Investigators trying to solve a giant arson wildfire outside Los Angeles have bolstered their ranks in recent days, putting six more homicide detectives on the case as they review dozens of tips from the public about who set the blaze that has ravaged a 250-square-mile stretch of national forest.

Investigators still have not found a culprit in the fire that erupted Aug. 26, burning 82 homes and killing two firefighters who died when their truck plunged down a ravine as they fled the flames.

Their deaths meant that as soon as the fire was identified as arson, the investigation became a homicide probe.

The fire's single point of origin has been traced to an area near an oak tree at the bottom of a steep ravine next to a curvy mountain road. Investigators reportedly found some kind of incendiary substance but have not disclosed more information. They ruled out lightning as a cause and said they found no glass that could have accidentally ignited a fire.

PHOTOS: California Wildfires Rage Near Los Angeles

The Los Angeles Sheriff's Department is the lead investigative agency, with 10 detectives now on the case — up from four since last week.

A reward of $150,000 is being offered and investigators want to hear from people with information about the arsonist, or anyone who saw unusual activity on the Angeles Crest Highway around 3:30 p.m. the day the fire broke out. The first reports of flames came shortly afterward.

About 50 people have called the department with possible information about who set the fire, Lt. Liam Gallagher said. He said they were treating all of them seriously and some might be more helpful than others.

"We get a lot of good people with good information, and your usual wingnuts, but each one we have to follow," said Gallagher, who is heading up the probe.

Investigators have left the crime scene, but the highway remains closed to the public and police tape still surrounds the roadside turnout near where the fire started.

Finding the ignition point was a crucial step in the case, said Tom Fee, a private arson investigator and business manager for the California Conference of Arson Investigators. Further progress will depend on the type of information provided by witnesses or evidence uncovered at the scene.

"It's a difficult task," said Fee. "The arrests that are made on arson fires are a very low percentage."

Guy E. "Sandy" Burnette, a Florida attorney specializing in arson litigation, said cases are solved as much by old-fashioned detective work as by modern forensics.

"People do stupid things," Burnette said. "They leave something behind. They say something to a friend, or do something that draws attention to themselves."

The blaze is still chewing its way through rugged backcountry terrain in the San Gabriel wilderness but was 71 percent contained as of Thursday.

Nearly 4,500 firefighters and 28 helicopters and airplanes were attacking the blaze, and firefighters took advantage of cooler weather overnight to set backfires Thursday near Mount Wilson to further protect a historic observatory there and several communications towers.

All the while, investigators are plodding away in search for a clue that could lead them to a suspect.

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