Even before the embers had cooled, investigators determined that a pyrotechnic device caused the Robbers Fire in Placer County. But, how do investigators find the source when everything around it has burned?
Robert Moore is an investigator for the Sacramento Fire Department. He says eyewitnesses provide the most information, but in really remote areas where no one sees the fire start, there are still clues to be had.
"Any time a fire moves through an area, not everything is just burned to pure ash," explains Moore. "There are heavier fuels that survive more intact than lighter fuels, and those fuels can have indicators of the direction that fire moved."
Moore says once he follows a fire back to its start, he looks for the source. If he's lucky, he'll find a burn barrel or a piece of equipment.
If the source of the fire includes an accelerant, the fire may burn much hotter at the beginning compared to how it burns in surrounding areas.
Moore uses a hand-held hydrocarbon detector to find out if a foreign substance was present when the fire started.
"They can detect traces of any type of hydrocarbon residue down to about three parts per million," says Moore. "A dog on the other hand is about ten times that sensitive. But they don't tell us what the fuel is. They just tell us that there's a trace there. We take a sample and send it to a lab."
According to the National Fire Protection Association, more than 15,000 fires are started by fireworks every year. Moore says a fire started by a firework is often the easiest to identify.
"If the area of origin isn't disturbed, it's pretty amazing what you can find," says Moore. "You can actually find the traces of the fuses, especially the cardboard tubes, and pieces of plastic or whatever is part of that firework."
According to Cal Fire, investigators were able to determine a cause for about three-quarters of all California fires last year.