Posted by: Don Penven, Technical Support Group
Arson investigators face considerable problems when investigating a fire scene. Unlike the majority of crime scenes frequently encountered by criminal investigators, a fire scene offers a completely different set of problems. Physical evidence generated by the perpetrator of crimes of homicide, robbery, rape and burglary is altogether different from what may be expected at a fire scene. Most of the physical evidence generated by the arsonist is often consumed by the fire.
An arson investigation begins with an understanding of the fire itself—the physics of fire if you will. The arson investigator will begin by questioning the first firefighters on the scene as well as witnesses, When materials burn they generate particular-colored flames and smoke. For example: wood produces a yellow or red flame with gray or brown smoke. On the other hand, gasoline and other hydrocarbons produce a yellow or white flame with black smoke.
But observing black smoke is not always a sure indicator that the fire was intentionally set. The structure may have had hydrocarbons stored there, like gasoline, kerosene, oil-based paint, acetone and paint thinners.
(Accelerant Burn Pattern on Concrete Floor)
Another point to clarify is to determine if any doors or windows were left open. And did any one notice any peculiar odors that might indicate that an accelerant was used. An accelerant is any substance that speeds the burning and spreading process of a fire. Accelerants include materials like hydrocarbons (gasoline, kerosene, etc.), paper, plastics, and other materials, which can cause a fire to spread more quickly or burn more fiercely than it would otherwise.
It is also important to determine whether any windows were covered to prevent the early stages of the blaze being seen from the outside.
For a fire to begin and sustain itself, three factors known as the “Triangle of Fire” must be met. The triangle of fire consists of oxygen, a fuel source and heat.
- The normal air present in most structures is sufficient to permit combustion. An arsonist may help this situation along by opening doors and windows.
- The fuel source may be simply surrounding flammable materials such as paper, furniture, wooden structures, etc., but the fuel source could be materials made available by the arsonist.
- And finally we need a heat source, Defective wiring is often cited as the probable cause of a fire since it may serve as a heat or ignition source, or the heat source may simply be a single match.
Once the fire has been completely extinguished and it is safe to enter the structure the investigator will attempt to locate the “low point” or point of origin. This is accomplished by examining the burn patterns. The charring of wooden surfaces can give a clue as to how intense the fire was as the wood will be charred. Intense heat causes deep charring or “alligatoring,” but this does not necessarily indicate that accelerants were used at a specific location.
We tend to think of a wooden beam, floor joist, door frame etc., as actually burning. But the physics of fire tells us that when wood is heated to high temperatures over a period of time it undergoes pyrolysis. In other words, the wood gives off a flammable gas that does the actual burning. The intense heat causes the charring of the wood.
In simple terms, fire is the visual manifestation of rapid oxidation. The principal byproducts of this oxidation are carbon monoxide and carbon dioxide.
The investigation generally begins with a walk-through of the structure. What one would expect to be present in a dwelling are items of furniture, appliances, TVs, computers, beds and bedding, clothing, etc.
In an office setting or retail store one expects to find other items like office furniture, file cabinets, and computers, while a retail establishment will have stocked shelves. If expected items are missing, Arson may have caused the blaze.
During the initial search the investigator will look for heat sources. The source could be electrical, chemical or intentional. Multiple hot spots are a dead giveaway, since this would be a rare occurrence without the aid of accelerants.
Fire scene investigation is a very complex process. The first goal of the investigator is to rule out that fire was accidental in nature. When all explainable sources are eliminated, then it is necessary to consider the fire as “suspicious in nature,” and look for human influences and actions that may have played a role.
Fire scene investigators undergo many months of intensive training, and the purpose of this and any future posts to follow is to provide the reader with an overview of what arson investigation is all about.
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Arson Scene Photos Courtesy of the Franklin Co. (NC) Sheriff's Dept.
Inter Fire Online, “The Pocket Guide to Accelerant Evidence Collection,” <http://www.interfire.org/res_file/aec.asp> January 31, 2011
International Association of Arson Investigators. User’s Manual for NFPA 921 2nd Edition. Sudbury, MA. Jones and Bartlett Publishers, 2005.