Preservation of Fire Scene Evidence
Trace evidence such as hairs and fibers, fingerprints and shoeprints, soils, blood, documents, etc. can be found anywhere at a fire scene. Finding the evidence can be more difficult at a fire scene than other types of scenes because of fire suppression activities and the condition of the scene. As a result, fire scene investigators will usually concentrate their efforts to find trace evidence in the areas where a perpetrator would most likely have been at the start of or during the fire. These would be the points of entry, exit, and origin. As explained previously, points of entry and exit tend to be remote from the point of origin of the fire and thus more likely to contain trace evidence that has been relatively shielded from the fire. The same precautions need to be obeyed when collecting trace evidence from fire scenes as with any other scene. The additional problem is that contamination with combustion products, fuels, and water makes it more likely that evidence will be adulterated or destroyed at fire scenes. The presence of an accelerant such as gasoline, especially around the point of origin of a fire, is generally indicative of the fire being incendiary. This must be put into context, however. A fire scene investigator will determine the type of fire based on observation of all of the factors that have been listed above, including the presence of an accelerant. Merely finding gasoline at a fire does not mean that the fire was arson or even incendiary. The gasoline could have become involved in the fire incidentally rather than purposefully. On the other hand, the absence of an accelerant where the conditions of the fire scene would seem to indicate its presence, does not rule out arson as a possible cause. Depending upon the duration and intensity of the fire, suppression efforts, and the nature of the accelerant employed, there may not be enough accelerant residue available to detect. Also, owing to extensive damage from a fire and collapse of debris from higher to lower areas of the building, the fire scene investigator may not be able to collect residues that may be present. Thus, a fire scene investigator may have ample evidence that a fire was arson and yet there may not be a finding of an accelerant by the laboratory. If an accelerant was present, it would be found most likely at the point(s) of origin or along fire trails. A number of methods are used to detect possible accelerants. The most sensitive method is the use of hydrocarbon-sniffing dogs. These dogs are specially trained to sniff out the smallest traces of hydrocarbons (the main constituents of fuels and accelerants) at fire scenes even after extensive burning and fire suppression. They are especially useful in finding traces of accelerant in
large volumes of fire scene residue. There are also analytical devices that can detect small amounts of hydrocarbons in a large amount of debris. These are essentially stripped-down gas chromatographs with a gas sensor. They are also quite sensitive. In jurisdictions that do not have dogs or hydrocarbon sniffers, the investigator relies on experience and observation to determine which fire residues are most likely to have trapped accelerant residues. For example, cloth materials such as bedclothes, clothing, carpeting, and upholstery are usually good sources of evidence because accelerants will soak into these materials and, in some cases, may be recovered intact even though there has been considerable fire damage. Hard, nonporous items such as flooring or wallboards are generally poor sources of evidence because accelerants will not soak into them and are thus easily burned or evaporated away. On the other hand, if there are seams in wooden flooring, for example, some of the accelerant may seep inside and be protected from combustion. When a determina tion is made of what samples to collect, it is very important that sufficient sample quantity be taken and that negative controls are also collected. In general, one can not collect too much sample, but there are, of course, practical considerations so the rule is to collect as much sample as is practical and likely to be fruitful. A nega tive control in this context is essentially the matrix where the accelerant residue is being collected. For example, if a partially burned carpet is suspected to contain accelerant residues, then some of the unburned carpet, far from the burned area to make sure that there is no unburned accelerant, should be collected. If possible, some of the burned carpet that is known to not contain any accelerant should also be collected. This is especially important when synthetic textiles are encountered because some of these may interfere with the chemical analysis of the acceler ant residues. The analyst must have a control sample of this material to aid in the analysis of the evidence.
