Almost every arson case rests on one thing: a fire investigator’s conclusion about how and where the fire started, and whether an accelerant was used. That conclusion arrives at trial wrapped in the authority of forensic science — but the science behind it is far more contestable than a police report makes it sound, and for decades much of what passed for “arson evidence” has turned out to be myth. If you are under investigation or have been charged with arson in Murrieta, Temecula, Menifee, Lake Elsinore, Wildomar, Winchester, Canyon Lake, or French Valley, the strength of the fire-origin evidence is where the defense begins. Call (951) 400-4357.
Arson is a serious felony, and these cases can involve real harm. But precisely because the stakes are so high, the evidence has to be held to the standard it claims to meet — and a surprising amount of it does not.
How Investigators Test for an Accelerant
When investigators suspect an ignitable liquid — gasoline, lighter fluid, a solvent — was used to set or speed a fire, they collect debris from the scene and send it to a laboratory. The standard testing process is well established and, done correctly, genuinely sensitive.
The sample is sealed in a clean metal can. A strip of activated charcoal is suspended in the headspace above the debris, and the can is gently heated. Any volatile residue evaporates and is adsorbed onto the charcoal strip. The strip is then rinsed with a solvent, and the resulting extract is run through a gas chromatograph–mass spectrometer (GC-MS), which separates the mixture into its individual chemical components and identifies them. This is the method described in the recognized forensic standards for fire-debris analysis (ASTM E1412 for the extraction, ASTM E1618 for the GC-MS identification), and it can isolate astonishingly small quantities — a fraction of a single drop.
So far, this is the prosecution’s evidence at its strongest. The problems begin not with whether the test can detect an ignitable liquid, but with what that detection actually means.
Finding an Accelerant Is Not the Same as Proving Arson
Here is the point that most surprises people, and it comes directly from the governing forensic standard itself: detecting an ignitable liquid residue at a fire scene does not establish that the fire was intentionally set. The ASTM standard that governs the GC-MS analysis says so in plain terms — the identification of an ignitable liquid residue does not, on its own, lead to the conclusion that a fire was incendiary, because further investigation can reveal a legitimate reason the residue was there.
And legitimate reasons are everywhere. Gasoline lives in garages and sheds. Charcoal lighter, paint thinner, mineral spirits, lamp oil, lawn-equipment fuel, nail-polish remover, and dozens of ordinary household and workshop products are ignitable liquids. A residue found in a home or garage may have been present long before the fire, entirely innocently. The lab result confirms a chemical was there; it does not tell you who put it there, when, or why — and the prosecution often treats that gap as if it were already filled.
The False-Positive Problem — Pyrolysis and Substrate Interference
The second problem is more technical and even more important: ordinary materials can produce chemical signatures that look like accelerants even when none was used. When carpet, flooring, adhesives, plastics, foam, and finishes are exposed to the heat of a fire, they break down and release pyrolysis and combustion products — and many of those products fall within the same chemical range as common ignitable liquids. A modern GC-MS analysis using extracted-ion profiling is specifically designed to tell genuine accelerant residue apart from this background interference, but it does not do so automatically. Interpretation of the data is widely regarded as one of the most difficult and subjective steps in the entire process, and it is performed by a human analyst making a judgment call. That judgment can be wrong, and it can be challenged.
This is why an independent review of the actual chromatographic data — not just the lab’s bottom-line conclusion — matters so much. Two qualified analysts can look at the same data and disagree about whether what they are seeing is an accelerant or the burned remains of the building itself.
The Discredited “Indicators” — Fire-Investigation Myths That Refuse to Die
For most of the twentieth century, arson was “proven” not by lab work at all but by an investigator’s reading of visual clues at the scene. Many of those clues have since been thoroughly debunked, yet they still surface in conclusions reached by investigators trained in the old methods. NFPA 921, the Guide for Fire and Explosion Investigations and the recognized standard in the field since 1992, has rejected indicator after indicator:
- Alligatoring — large, shiny, rolling char blisters on wood — was long treated as a sign of a fast, accelerant-fed fire. It is not. Char appearance bears no scientific relationship to whether an accelerant was used.
- Crazed glass — fine cracks in window glass — was read as evidence of rapid heating from an accelerated fire. In reality it is typically caused by rapid cooling, when firefighting water hits hot glass.
- Depth of char and burn patterns are shaped by ventilation, fuel distribution, and the layout of the structure, not by the presence of a poured liquid.
- Irregular “pour patterns” and burn-through holes in floors were once taken as proof that someone splashed accelerant around a room. But once a fire reaches flashover — the point at which an entire room ignites — these patterns appear naturally, produced by ventilation, burning debris, structural collapse, and the firefighting itself. Attributing them to an accelerant in a post-flashover fire is not scientifically defensible.
Underlying all of it was a single false premise — that accelerated fires burn hotter than ordinary ones. They do not; they release heat faster, but reach the same temperatures. An investigation built on these discredited indicators is building on sand, and exposing that is a central part of the defense.
Negative Corpus — Calling It Arson by Process of Elimination
There is one more flawed method worth understanding, because it underlies many weak arson cases. “Negative corpus” is the practice of concluding a fire was deliberately set simply because the investigator ruled out the accidental causes they could think of — without any affirmative evidence of an incendiary origin. NFPA 921 rejects this reasoning outright as inconsistent with the scientific method, because it rests on an untestable hypothesis: “I can’t explain it, therefore someone set it.”
That is not proof. A fire whose cause genuinely cannot be determined is an undetermined fire, not an arson fire. The prosecution must come forward with affirmative evidence that the fire was intentionally and maliciously set — and where its case actually rests on elimination dressed up as a conclusion, that is a vulnerability, not a strength.
How the Defense Challenges Arson Evidence
Put together, these issues define the defense work in a fire case. When we review an arson prosecution, the cause-and-origin evidence is the first thing we examine, and the questions are concrete: Did the investigation actually follow NFPA 921, or does it rely on discredited indicators? Is the cause-and-origin conclusion built on affirmative evidence, or on negative-corpus elimination? Does the lab’s GC-MS data, independently reviewed, really support the accelerant classification — or could it be substrate interference? Is there an innocent explanation for any residue found? Was the evidence collected, stored, and handled without contamination? Bringing in an independent fire-origin expert and an independent analyst, early, is often what turns the case.
This connects directly to what the prosecution must prove. Arson under Penal Code § 451 requires a willful and malicious burning. If the fire-origin evidence cannot establish that the fire was intentionally set, the § 451 charge cannot stand — the case may collapse entirely, or fall to the far less serious offense of reckless burning under Penal Code § 452, which carries no arson registration. The statute, subdivisions, penalties, and the lifetime arson-registry consequences are covered in depth in our PC § 451 arson defense materials.
How Arson Is Investigated in Southwest Riverside County
In a fire-prone region like ours, fire investigations are worked aggressively. Wildland and structure fires across Murrieta, Temecula, and the surrounding communities are typically investigated by CAL FIRE and Riverside County Fire investigators, whose cause-and-origin findings drive the charging decision, and the cases are filed by the Riverside County District Attorney at the Southwest Justice Center in Murrieta. Because the investigation that decides a case often begins at the scene — long before charges are filed — the early decisions matter enormously: whether the scene and the evidence are properly preserved, and whether an independent fire expert is brought in before the prosecution’s narrative hardens into a charge. Our office has defended fire cases throughout Southwest Riverside County and appears regularly at the Southwest Justice Center.
Talk to a Murrieta Arson Defense Attorney
An arson charge can rest on forensic conclusions that look authoritative and turn out to be contestable — an innocent source for a detected liquid, a substrate that mimicked an accelerant, a conclusion built on debunked indicators or on ruling out accidents rather than proving a crime. The earlier those questions are raised, the more room there is to change the trajectory of the case. The Law Office of Nic Cocis defends arson and fire-related charges across Murrieta and Southwest Riverside County. Contact our office or call (951) 400-4357.
