Once the investigator has obtained information regarding the evidence that the fire fighters observed and a check of the financial records is complete the investigator will examine the results of the physical evidence found once the fire was extinguished. When examining the physical evidence the investigator must be able to rule out accidental causes of the fire in order to establish that the fire was deliberate (Battle, 1978).
Some of the most common causes of accidental fires are faulty gas or electrical equipment improperly stored material, children, pets, smoking, stoves left on and lightning. Plumbers, Electricians and Gasfitters may be asked to examine these systems for signs that they were faulty (Lane, 1992) These types of fires usually show one obvious point of origin as opposed to multiple points of origin or no detectable point of origin as with an arson fire (Lane, 1992)
Several things are looked for when examining the scene for signs of accelerant use. The first and most obvious sign is the presence of containers in areas where accelerants would not normally be found (French, 1979) ( Kennedy , 1977). A hydrocarbon vapor detector can be used to do the initial examination of the area in question. Once the presence of hydrocarbons has been detected further sampling must be done and sent to the lab for analysis (French, 1979)(Saferstein, 2004).
Liquid when poured on a carpet will soak into the carpet creating a concentrated char pattern on the sub-floor below. Investigators will pull up carpeting and examine the surfaces below the carpet. Samples of the carpet and the floor below will be sent to the lab for analysis (French, 1979) (Kennedy, 1977). Liquid poured on hard surfaces tends to flow. The underlying sub-floor should be examined for charring showing flow patterns. The baseboards and walls should be examined for char patterns indicating liquid was splashed on them. The baseboards and underlying sub-floor should be examined for sign of seepage. Floors below the suspect area should be examined and sample taken. Samples should also be taken of the soil under the foundation (French, 1977)
When samples are taken they are placed in airtight containers. An older method of testing was to withdraw a sample of the air in the ‘headspace’ of the container and test is using gas chromatography (Saferstein 2004). A newer method is to place a strip of activated charcoal in the container. The container is heated to allow the vaporization of any hydrocarbon residue. The vaporized residue is trapped in the charcoal. This method allows the analyst to test much smaller concentrations of residue since it is being concentrated with in the charcoal (Saferstein, 2004)
Incendiary devices can be as simple as a lit match and as complex as a timed system utilizing and electric current imaginable. Common timed devices are candles or lit cigarettes attached to a book of matches (Kennedy, 1977) Cell phones, computers and thermostats can all be rigged to set off ignition of a fire when activated. The key to these devices is for the investigator to determine if they were likely to cause a fire and is the location reasonable. Candle residue in a hallway or closet is not reasonable and raises suspicion. The point of origin must match the ignition source and the ignition source must be in a place in which it would not normally occur to prove an arson has been committed (Battle, 1978).
Arson investigation occurs in a different order than the usual criminal investigation. However the same steps are covered and it is the job of the investigator to analyze all of the information and come to a conclusion. The arson investigator most act swiftly in conducting interviews because fire may destroy physical evidence making witnesses all that much more important. A combination of interviews, background information and physical evidence is used to determine whether a fire is the result of arson or not.