Arson is difficult to investigate, because…
- Arson can be planned in advance.
- Arson can occur without the presence of the arsonist.
- Arson can destroy important evidence related to the crime.
Signs of arson include…
- Unusual activity before or during fire
- Multiple ignition sites
- Fire moved unusually quickly
- Occurrence of flashes and explosions
- Unusually-colored fire or smoke
- Gasoline produces yellow or white flame with black smoke, and wood produces a yellow or red flame with brown or gray smoke, so a burning wooden house that produces lots of black smoke suggests the presence of lots of gasoline.
- The majority of the burning on the floor rather than the ceiling, indicating the presence of accelerant or fuel on the floor.
- Notable absences, including
- Potential accidental causes
- Valuable items, including items of sentimental value
- People who might have been harmed by the fire
- Environmental factors
- Evidence of forced entry
- Security systems (e.g. smoke alarms, sprinklers) were sabotaged
- Doors and windows propped open to improve ventilation.
- Alternately, doors and windows are secured shut to prevent firefighters from accessing them.
- Lines of accelerant residue, indicating that it was poured or set down.
- Unconsumed combustible liquids remaining on site.
- Presence of the same person at multiple unrelated sites (arsonist may like watching the fire or helping the firefighters).
- Presence of unconsumed accelerants, or a lingering odor from accelerants (e.g. “hydrocarbon odors” from gasoline, kerosene, etc.).
Collecting evidence of arson includes…
- Taking samples of ash, soot, and other porous materials, and storing them in airtight containers. They may contain unburnt accelerant.
- Using a vapor detector (i.e. sniffer) to collect and identify vapors, or trained dogs.
- Taking and examining burnt material, because different accelerants burn at different temperatures.
- Searching for obvious ignition devices like electronic ignitors, matches, or glass (from a Molotov cocktail).
- Talking with the firefighters that arrived on scene.
The fire triangle refers to the three things which are necessary to create and sustain a fire:
- Oxygen (typically above a concentration of 16%)
- Fuel source
Fires aren’t happening all over the damned place, so most locations in the world do not satisfy the fire triangle. Arsonists therefore have to introduce or modify one of the factors (e.g. by adding a fuel source), and arson investigators look for ways that the fire triangle was completed.
Fire burns up and out, so the presence of a “V” pattern may lead investigators to the origin of the fire. Fire first travels up, then expands outward when it meets resistance (i.e. a ceiling), producing a cone which, against a flat vertical surface (i.e. wall) creates the aforementioned “V” pattern. An inverted “V” pattern is a sign of arson, because it indicates that the fire expanded outward (following a mass of accelerant) before it expanded upward. Because a location is likely to have many flammable substances, the site of a fire may have multiple “V” patterns if the fire burned slowly, but a fast-burning fire (such as one deliberately fed by heavy accelerants) will be unlikely to have many “V” patterns, inverted or otherwise. When the ceiling begins to burn, this is called a “flash over,” and it can obscure the nature of the “V” pattern and whether it was inverted. Firensics refers to this as “fire language.”
it can sometimes be difficult to correctly identify the origin or nature of a fire. If a copper pipe in an air conditioner heats to a high temperature, then it may conduct that heat to a more flammable substance. Radiated heat can, very rarely, produce straight lines that might be mistaken for signs of deliberately-spread accelerant.
Pyrolysis is the “thermal decomposition” of burning substances. Not everything pyrolyzes at the same rate, or releases the same amount of heat when it does.