In the world of fire investigations, incorrect cause determinations resulting from misconceptions surrounding the evidence have occasionally led to wrongful convictions.
Often, these errors can be attributed to “negative corpus” thinking, a methodology where investigators come to a determination by eliminating possible ignition sources based on a lack of evidence. This can result in the investigator potentially overlooking alternative explanations and making a determination based on their own preconceived notions. This approach can lead to a narrow focus on collecting supportive evidence, neglecting crucial information that might challenge the original assumption.
The scientific method ultimately forms the basic principle for any fire investigation: recognise the need, define the problem, collect and analyse data, develop and test hypotheses, and select final hypotheses. It is imperative for investigators to be aware of any misconceptions about the origin and cause of a fire and approach each case with a meticulous, science-based methodology.
Critical Evaluation of Evidence is Key
Fire investigators are guided by National Fire Protection Association (NFPA) codes and standards, specifically NFPA 921, Guide for Fire and Explosion Investigations. However, as our understanding of fire behaviour and chemistry advances, much of the guidance in the early editions of NFPA 921 has become myth. The most recent edition of Scientific Protocols for Fire Investigation, authored by highly regarded fire investigator John Lentini, details updated practices through case studies and research.
Specifically, investigators should avoid negative corpus thinking and be aware of any possible misconceptions they might harbour about a fire’s development and behaviour before making a cause determination. This includes critically evaluating:
- Pooling. The term pooling has often been used to describe the presence of an ignitable liquid. The use of this term is both misleading and incorrect. Irregular patterns can be created following post-flashover conditions, structural collapse, or pro-longed firefighting activity. Whilst the pooling of an ignitable liquid may be one reason for such patterns, irregular patters can also be generated as a result of hot gases, smouldering or flaming debris and melted plastics. If there is any suspicion that an ignitable liquid has been deliberately used to accelerate the fire, then samples must be taken and sent for analysis.
- Crazed glass. The presence of crazed glass at a fire scene has also been interpreted as an indicator of rapid fire development induced by the presence of an ignitable liquid. The reality is far different, the crazing of glass is caused by rapid cooling during firefighting operations as the water makes contact with the glazing.
- Depth of char. While char patterns are often used to determine the length of burning and origin, they can also be influenced by factors such as ventilation, fuel distribution or the structural layout, challenging the assumption of a deliberate act. The term alligatoring has been used to describe the appearance of rounded char blisters that develop as the wood burns. There have been occasions in the past when large shiny blisters were regarded as being an indicator that an ignitable liquid was used. This is untrue. A variety of timbers exposed to the same fire conditions will display char blisters of different sizes, challenging the assumption of a deliberate act.
- Lines of demarcation. The perceived boundaries between burned and unburned areas can mislead investigators about the fire’s origin and spread. For example, an irregular shaped (to the lay person appearing puddle-shaped) line of demarcation, on floors or rugs, could be perceived to indicate the use of an ignitable liquid. This was widely accepted in some courts as evidence of the presence of ignitable liquids.
- Ignitable liquid detection dogs can be of great assistance to the fire investigation. Trained dogs are capable of sniffing out minute amounts of ignitable liquids used at fire scenes.
- Dog detection. Ignitable liquid detection dogs are trained to sniff out minute amounts of ignitable liquids used at fire scenes. Some courts have accepted a positive identification from a detection dog as evidence of the presence of ignitable liquids. However, responses from detection dogs should be treated as presumptive results with the potential for false positives. The investigator should take samples from the area to confirm these results with an accredited fire debris analysis laboratory.
- Positive lab results. Laboratory analyses of certain ignitable liquids can result in false positives, as the same compounds can be found in common household items. The presence of certain compounds in a sample is not always conclusive that an ignitable liquid was deliberately used to accelerate the fire.
Experienced Fire Investigators Rely on Facts
To prevent inaccurate origin and cause determinations, it is imperative for fire investigators to be aware of these misconceptions and approach each case with a meticulous, science-based methodology.
Hiring an investigator who has expertise in scientific methodology and conducting thorough analyses of fire scenes is key to producing an accurate cause determination. Jensen Hughes forensic fire investigators draw upon their experience and training having investigated thousands of fires, to ensure the facts are gathered, confidently delivering on reliable origin and cause determinations.
