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Did He Start the Fire?How arson investigations work.

By Daniel Engber
Posted Friday, Nov. 3, 2006, at 6:07 PM ET

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Prosecutors filed charges on Thursday against Raymond Lee Oyler, the man suspected of setting last week's deadly Esperanza fire in Southern California. Oyler faces multiple counts of murder, arson, and use of an incendiary device. How do you investigate a wildfire for signs of arson?

First, figure out where it got started. The place where firefighters first engaged with the blaze is a good place to begin, as are spots where eyewitnesses say they first saw flames or charred ground. Once they're in the ballpark, a careful study of burn patterns can guide investigators to the fire's point of origin.

The patterns are helpful because a fire usually burns up and away from its origin. Nearby grass will be singed in that direction, with burns that taper away from the source of the flames. Trees are more likely to be charred black on the side from which the fire advanced, with less damage on the opposite face. Noncombustible items—like rocks, soil, beer cans, or rabbit pellets—may show patterns of discoloration that suggest a fire moving in a particular direction.

Once the investigators have narrowed the origin of the fire to a small enough area, they can lay down something like an archaeological grid and start sifting through the debris. The top level of fire debris may need to be blown off to find the physical evidence beneath. This evidence might include the "puddled" burn patterns caused by an accelerant, or the remains of a cigarette. Even a large fire could spare a butt or the bottom end of a match, especially if a strong wind carried the fire away from its source. Investigators also look for footprints or tire marks, and they sometimes use magnets to find stray bits of metal that might have been part of a time-delayed incendiary device. (Not every incendiary device has metal parts—the simplest consist of a lit cigarette bound together with a book of matches.)

Why do investigators suspect arson in the first place? In some cases, they use negative evidence. They are likely to attribute a blaze to foul play if they can't figure out any way in which it could have started naturally. There's something fishy about a fire that starts out in the open without any record of lightning in the area. That's one reason police suspected arson as the cause of the Esperanza fire. An eyewitness also said he saw a couple of men leaving the scene around the time the fire started.

Investigators in California can consult a registry of convicted arsonists to see if any easy suspects happen to be living nearby. Unlike the registry of sex offenders (which is run by the same program), the arson registry is only accessible to members of law enforcement. California also maintains a database of arson cases that includes a list of suspects and a modus operandi for each fire. The five choices for M.O. are liquid accelerant, nonliquid accelerant, heat source, trailer, and incendiary device.

Got a question about today's news? Ask the Explainer.

Explainer thanks Dick Ford of the International Association of Arson Investigators and Brad Hamil of Hamil Investigations.

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Daniel Engber is an associate editor at Slate. He can be reached at .
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