Arson investigations often present complex and difficult circumstances to investigate due to the fact that the perpetrator has thoroughly planned the act, is not present during the act, and the destruction is so extensive. The criminalist’s function is rather limited to detecting and identifying relevant chemical materials collected at the scene and reconstructing and identifying igniter mechanisms.
Arson is the second leading cause of death by fire in the United States – topped only by smoking – and the main cause of property damage due to fires. Arsonists killed more than 500 Americans in 1996 and inflicted property damage totaling more than $2 billion. Arsonists often escape punishment. Only 16 percent of arson offenses lead to arrest, and only 2 percent of those arrested are convicted. In 1996-the last year for which full data is available-those 20 years of age and under accounted for 62.3 percent of all arson fires in the United States. Of that total, 35.5 percent were under the age of 15. Each year, more than 90 percent of all civilian deaths in incendiary and suspicious structure fires typically occur in residential properties, principally homes. There appears to be a growing link between arson and illegal drug activity. Preliminary results of a new study by the National Fire Protection Association suggest that between 20% - 25% of reported arson cases in major American cities are drug-related.
1996-the last year for which full data is available-those 20 years of age and under accounted for 62.3 percent of all arson fires in the United States. Of that total, 35.5 percent were under the age of 15. Each year, more than 90 percent of all civilian deaths in incendiary and suspicious structure fires typically occur in residential properties, principally homes. There appears to be a growing link between arson and illegal drug activity. Preliminary results of a new study by the National Fire Protection Association suggest that between 20% - 25% of reported arson cases in major American cities are drug-related.
Chemically, fire is a type of oxidation, which is the combination of oxygen with other substances to produce new substances. To start fire, the minimum temperature needed to spontaneously ignite fuel, known as ignition temperature, must be reached. The heat evolved when a substance burns is known as heat of combustion. An additional factor, besides the liberation of energy, needed to explain fire is the rate or speed at which the oxidation reaction takes place.
A fuel will achieve a reaction rate with oxygen sufficient to produce a flame only when it is in the gaseous state. A liquid burns when the temperature is high enough to vaporize it (flash point), while a solid must be hot enough to decompose into gaseous products (pyrolysis). Glowing combustion or smoldering is burning at the fuel-air interface, such as a cigarette. Spontaneous combustion, which is rare, is the result of a natural heat-producing process in poorly ventilated containers or areas.
The three mechanisms of heat transfer are conduction, radiation, and convection. Conduction is the movement of heat through a solid object. Radiation is the transfer of heat energy by electromagnetic radiation. Convection is the transfer of heat energy by the movement of molecules within a liquid or gas.
The arson investigator needs to begin examining a fire scene for signs of arson as soon as the fire has been extinguished. Experience shows that most arsons are started with petroleum-based accelerants. The necessity to begin an immediate investigation even takes precedence over the requirement to obtain a search warrant. The search of the fire scene must focus on finding the fire’s origin, which may be most productive in any search for an accelerant or ignition device. such as matches, an electrical sparking device, or parts of a “Molotov cocktail” must also be conducted.
Some telltale signs of arson include evidence of separate and unconnected fires, the use of “streamers” to spread the fire from one area to another, and evidence of severe burning found on the floor as opposed to the ceiling of a structure, due to a flammable liquid. Normally, a fire has a tendency to move in an upward direction, and thus the probable origin will most likely be the lowest point showing the most intense characteristics of burning. Fortunately, combustible liquids are rarely entirely consumed during a fire.
At the suspect point of origin of a fire, ash and soot, along with porous materials which may contain excess accelerant, should be collected and stored in airtight containers, leaving an airspace to remove samples. Traces of flammable liquid residues may be located with a vapor detector (sniffer). It is important that a sampling of similar but uncontaminated control specimens be collected. A search for ignitors such as matches, an electrical sparking device, or parts of a “Molotov cocktail” must also be conducted.
The easiest way to recover accelerant residues from fire-scene debris is to heat the airtight container in which the sample is sent to the laboratory. When the container is heated, any volatile residue in the debris is driven off and trapped in the container’s enclosed airspace. The vapor or headspace is then removed with a syringe. When the vapor is injected into the gas chromatograph, it is separated into its components, and each peak is recorded on the chromatogram.
In the laboratory, the gas chromatograph is the most sensitive and reliable instrument for detecting and characterizing flammable residues. The vast majority of arsons are initiated by petroleum distillates such as gasoline and kerosene. The gas chromatograph separates the hydrocarbon components and produces a chromatographic pattern characteristic of a particular petroleum product. By comparing select gas chromatographic peaks recovered from fire-scene debris to known flammable liquids, a forensic analyst may be able to identify the accelerant used to initiate the fire.
Vapor concentration: - technique a charcoal strip is placed in the airtight debris container when it is heated. The charcoal strip absorbs much of the vapors during heating. The strip is washed with a solvent which will recover the accelerant vapors. The solvent is then injected into the gas chromatograph for analysis.
Complex chromatographic patterns can be simplified by passing the separated components emerging from the gas chromatographic column through a mass spectrometer. As each component enters the mass spectrometer, it is fragmented into a collection of ions. The analyst can then control which ions will be detected and which will go unnoticed such that the mass spectrometer acts as a filter allowing the analyst to see only the peaks associated with the ions selected for a particular accelerant
Chapter 15 Forensic Aspects of Fire Investigation