By Todd J. Hedglin, CFI, CFEI
Try to remember back to the first time you played on a sports team, the first day of high school, or even the day you chose to join the military. The first day always requires direction from someone – the coach, a teacher, your drill instructor. The US Marine Corp has a methodical approach to training new recruits from the day you step off the bus to the day you receive your discharge paperwork years later. Often, straying from that methodical approach may result in a recruit being assigned multiple push-ups to help them remember the process the next time.
The Marine Corp teaches the same subject, such as weapons and tactics, to each marine the same way every time. In turn, they produce marines who are able to function the same way no matter the unit or the mission they are given.
Let’s apply the same theory to fire investigation. Typically, investigators should perform a fire investigation the same way each time they go to a scene. As a general rule, the investigation should start from the outside and move to the inside, or work from areas of the least damage to those of the most damage, in order to establish where the fire started. Witness information, fire patterns, arc mapping, and fire dynamics can all be used to assist in determining the area of origin.
Without knowing the origin, there is no way to determine the cause of the fire. The investigator must be able to describe the ignition sequence, as defined by NFPA 921 – Guide for Fire and Explosion Investigations as, The succession of events and conditions that allow the source of ignition, the fuel, and the oxidant to interact in the appropriate quantities and circumstance for combustion to begin. Using the scientific method provides a methodology, as well as some recourse for the investigator, if something is missed during the investigation or new information becomes available. While the systematic approach will not guarantee a correct answer every time, it will provide a framework to steer the investigation down the right path.
Once the area of origin has been identified, all ignition sources in that area should be considered. As part of the scientific method, investigators use the process of elimination to include or exclude potential ignition sources and the role they play in causing the fire. In the past, investigators have used the process of elimination to rule out ignition sources until only one remained, and then came to the conclusion that the sole remaining ignition source was the cause of the fire with no testing or evidence to support such a claim. This process was known as negative corpus, which means “without the body of the crime.” Many fires in the past have been called based on this methodology, often to the detriment of the “arsonist” wrongly convicted of a crime.
NFPA 921 defines negative corpus as, “The process of determining the ignition source for a fire, by eliminating all ignition sources found, known, or believed to have been present in the area of origin, and then claiming such methodology is proof of an ignition source for which there is no evidence of its existence.”
Initially, the true purpose of negative corpus was to allow the investigator to determine the cause of a fire where the origin was clearly defined and it was believed that the evidence of an ignition source was either removed from the scene (by the fire setter) or destroyed by the fire or subsequent suppression activities.
One problem lies in the term “clearly defined” when describing the area of origin, as some investigators have taken a liberal view on what constitutes an obvious area of origin. The conclusions as to the origin and cause of a fire should be based on evidence, (i.e. fire patterns, witness statements, fire dynamics, and arc mapping), and not the absence of evidence. Similarly, the ignition source selected should be based on the same info. While negative corpus has more often been used to determine a fire was intentionally set, it is also possible to conclude a fire was electrical in nature using the same process. Again, the lack of evidence does not constitute evidence in itself, and just because components from a specific appliance are missing does not mean they “vaporized” while starting the fire.
It is important to remember an investigator cannot use any single tool in the “tool box” as the only method for determining the origin or cause of a fire. You cannot base all of your conclusions for the cause of a fire on a single witness statement, a single burn pattern on the wall, or arc mapping alone. It is a combination of these tools and methods that provide the most comprehensive method for determining origin and cause. Any cursory search on the internet will come up with numerous papers discussing the pitfalls of this methodology, as well as court decisions that were made because this inappropriate methodology was utilized.