A streak of soot on a rock. Singed bark on a tree. Charred plants and grasses.
Even in a landscape blackened by wildfire, clues abound for investigators following the path of a blaze back to its point of origin and trying to find out how it started.
In the hills above Los Angeles, a team of U.S. Forest Service investigators is undertaking that hunt as they work to learn what, or who, sparked one of the largest wildfires in Southern California history.
The blaze has claimed the lives of two firefighters, ravaged more than 250 square miles, destroyed more than 60 homes and continues to chew through a forest normally enjoyed by Los Angeles residents escaping the sweltering city.
Authorities have provided scant details about how the blaze began, but its origin is being treated as a potential crime scene.
Deputy incident commander Carlton Joseph said the fire was human-caused, though he and other officials later backtracked and said they are looking at all possible causes.
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," Tunnell said. "We can follow fire progression back to the point at which it started."
The key for investigators is to pinpoint that origin as fast as possible. They start at the place firefighters were first called, then interview witnesses and look for indicators on the ground to work backward to the fire's place of ignition.
"You just follow your burn patterns," said Tunnell, a veteran of 50 wildfires who is based in Ukiah in Northern California. Clues can come from burned trees and grasses, where the amount of burned foliage tells investigators the direction and speed a fire was moving.
Once they find the general origin of the fire, investigators set up a perimeter and search the area in a grid formation until they find the actual place of ignition, Tunnell said.
If investigators are lucky, the point of origin will not have been disturbed by firefighting. Sometimes efforts to douse the flames inadvertently destroy potential evidence with water or the machinery used to cut fire lines.
It is not known if that is the case in this fire. A trio of Forest Service investigators wearing black gloves spent most of Wednesday scouring the area around a partially burned oak tree along a highway.
One investigator shook soil into a can, others used binoculars to examine the ground more closely. They planted red, blue and yellow flags along a burned-out rocky slope that climbed about 40 feet above the road. About 30 yards away, a smaller area was cordoned off with tape and orange cones.
"We look for something that is not supposed to be there," said Forest Service spokesman Nathan Judy. "Something out of the ordinary - is there a cigarette there? A party spot, debris, kids out there with fireworks?"
At the point of origin, investigators are often able to find the remains of whatever started the fire: a charred match or cigarette butt, a piece of metal from a car, a piece of power cable. If no such object is found, investigators will often rule a fire to have been "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," Tunnell said.
Almost anything can turn out to be a vital clue. In November 2007, a fire in Malibu destroyed more than 50 homes, 35 other structures and burned more than 4,900 acres. Weeks later, five men were arrested after investigators found precut fire logs and discarded food wrappers by a cave where an illegal campfire had been started.
By checking cash register receipts at a local store, investigators found the people most likely to have bought the logs and food, and used debit card records to track them.
Most wildfires are caused by human activity of some sort. The only exceptions are those caused by lightning and volcanoes. 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.
At the time the current fire broke out, Forest Service officials said there was no lightning and no power lines in the vicinity.
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 found at several fires.
They recovered everything from simple paper matches to a "layover" that consisted of matches balanced on a single cigarette 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.
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Associated Press writers Greg Risling, Jacob Adelman and Raquel Maria Dillon contributed to this report.
