Written by Katherine Steck-Flynn
Arson investigation starts with the fire itself. To create and sustain a fire three factors must be present. The three factors are known as the fire triangle (Peige ed., 1977). The fire triangle consists of oxygen, a fuel source, and heat. In most cases the percentage of oxygen concentration must be above 16% (Peige, ed., 1977). The fuel may be any flammable substance. The heat source needs only to match the ignition temperature of the fuel.
In a fire involving arson the arsonist will have tampered with one or more of the factors in the fire triangle. The arsonist may increase the fuel load by introducing flammable material or by adding accelerants such as kerosene, gasoline or alcohol (French, 1979) The arsonist may increase the oxygen content of a structure by opening windows or punching holes in ceilings and walls (French, 1979) Fire will follow the highest concentration of oxygen to its source. By ventilating a structure at the top and starting a fire at the bottom of the structure an arsonist can cause the fire to race upward through the structure. The fire will rapidly involve the whole structure rather than be confined to one room.
An arson fire involves the introduction of a heat source that can be as simple as a match or as complex as chemicals with very low ignition temperatures (Battle, 1978). By definition a fire is considered an arson fire when all other accidental caused have been ruled out. To say that the cause of a fire was arson and therefore deliberate, the investigator must have sufficient evidence the one of the factors in the fire triangle was tampered with.
The Fire Fighters
Arson investigations start with the observations of the fire fighters who respond to the scene. The fire fighters may not see any obvious signs of arson but may get a feeling that the fire is not behaving normally (Kennedy, 1977) (Battle , 1978) ( French , 1979).
There may be obvious signs of arson such as multiple points of origin or the presence of accelerants. What ever it is that raises the suspicions of the fire fighters at the scene it is their observations that initiate an arson investigation.
At this point the investigation deviates from the procedures used in a normal criminal investigation. In a normal criminal investigation interviewing witness is usually done after the physical evidence is processed (Osterburg & Ward, 2000). In an investigation of a fire where arson is suspected the investigator starts the interviewing of witness and fire fighters immediately (Kennedy, 1977). It is essential for the investigator to interview the fire fighters who were the first to arrive at the fire and those who fought the fire. Fire will often destroy evidence of arson. As well, the water and chemical foam used by the fire fighters may destroy evidence. The first fire fighters on the scene may have noticed evidence that may not exist by the time the fire is extinguished. The arson investigator will ask specific questions aimed at revealing suspicious conditions with in the structure and around the fire scene. The arson investigator will ask the following questions.
What was the color of the smoke? Different substances produce smoke in a variety of colors (Lane, 1992). The color of the smoke can indicate what is burning. The investigator can then determine whether a particular substance would normally be found with in the structure. For example, gasoline produces a yellow or white flame with black smoke (Lane, 1992). Wood produces a yellow or red flame and grey or brown smoke (Lane, 1992). If a burning wooden house is producing thick black smoke this could indicate the presence of gasoline. A large amount of gasoline being stored in a house is unusual and raises the suspicion that gasoline was used as an accelerant.
Were there inoperative hydrants, standpipes, alarm systems or sprinkler systems? If there were the investigator will direct plumbing and other trades experts to examine the systems for signs of damage (French, 1979) (Kennedy, 1977).
What was the condition of the doors and windows? Signs of forced entry can indicate an actual break and enter or indicate a staged scene. Doors and windows propped open could indicate an attempt to ventilate the building allowing the fire to spread through out the structure. Doors and windows that have to be forced open could indicate an attempt to hinder the fire fighters. Doors and windows that have been blocked out could indicate an attempt to conceal the fire until it is fully involved (Peige, ed. 1977)
Was there anything strange about the way the fire was burning? Rapid spread of a fire can indicate the use of accelerants. Multiple points of origin, flashes and explosions can also indicate accelerants or incendiary devices (French, 1979) (Peige, ed., 1977)
Where there any familiar faces at the scene? It is possible that the fire fighters have seen the same person in the crowd at several fires. Some arsonists like to watch the fire or help the firefighters. If the fire fighters do notice the same person at several suspicious fires the investigator must try to interview this person as so as possible (Kennedy, 1977).
Did the structure contain business equipment or household items normally expected to be present?
The absence of normal household items or business equipment can indicate that they were removed before the fire started. Removal of these items can indicate that the fire was planned. Removal of sentimental items such as baby picture, school records and valuables can indicate a planned fire. Evidence of pre-planning a fire gives the investigator excellent proof of intent to commit arson.