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Fire Investigation 101: A Forensic Expert’s Process and Mentality

August 15, 2022

Adjusters often call us in shock, wondering how we could look at a burnt pile of rubble and determine the cause of a fire so decisively. The truth is, a Forensic Expert’s most valuable tool isn’t their investigative intuition. Rather, they adhere to a strict investigative process called “The Scientific Method.”

Here, we take you through the steps of a fire and explosion investigation using a real-life case study. This gives you a glimpse inside the mind of a forensic investigator and shows how they use the scientific method to navigate through a burnt pile of rubble to find the needle in the haystack.

Going from a charred mess to an origin and cause determination

All the information discussed here can be found in the NFPA 921 guide for fire and explosion investigations. This publication sets the bar for scientific-based investigation and analysis of fire and explosion incidents. The extent to which the Canadian courts recognize this guide as authoritative is ever-increasing.

To be successful in court, your forensic expert must show that a non-biased systematic approach was adhered to. The systematic approach used in all physical sciences, including fire investigation, is based on the scientific method. Only by consistently applying the scientific method to every investigation can the investigator substantiate and defend their origin and cause determination.

The scientific method

This method entails steps for both fires and explosions. As an investigator, you must identify the problem, collect data, analyze it, develop, test, and select a final hypothesis. After a fire or explosion, it is our job to determine what happened. First, we need to know where and how the fire or explosion originated. All the facts are gathered and documented during data collection.

Figure 1: An illustration of steps involved in the Scientific Method.

To analyze the data, we must understand and attribute meaning to the data we’ve collected, develop hypotheses based on the data, and then develop explanations for the event. We must then compare each hypothesis to all the known facts.

The testing is designed to refute the hypothesis rather than to prove it. This process prevents relying on evidence that only supports the hypothesis. Instead, investigators must conduct a review of the entire process, ensuring that all critical data is accounted for and that all alternative hypotheses have been considered and eliminated.

Identifying and defining the problem

The various activities required to determine the origin and cause using the scientific method occur continuously and sometimes simultaneously. The first step of the scientific method is when the adjuster requests our assistance in a fire investigation. They provide general information. Preplanning at this stage can increase efficiency and, therefore, the chances of success for the overall investigation.

Key considerations at this step:
  • Are there any safety concerns?
  • Do I need heavy equipment or other specialized tools?
  • Do we need any other experts, such as engineers?
  • Are there security considerations for the site? For example, do we need fencing or 24-hour security to maintain the same continuity?
  • Have all interested parties been given reasonable notice of the scene exam, and have they been allowed to participate in the investigation?
Data collection

This step involves gathering and documenting all the facts about the incident. Some of the activities at this stage include witness and occupant interviews, examination of the scene, evidence recognition and collection, photography, site diagrams, and research, among others.

For both fires and explosions, the methodology is the same regardless of the size or complexity of the incident.

Case study

Figure 2: The home before the fire, as shown on Google Streetview.

This case study is from a fire loss involving a residential structure. The structure was a one-story single-family residence with a single attached garage.

Examining the scene

The fundamental purpose of conducting a scene exam is to collect all the available data and document the scene. Once the structure is torn down or repaired, any overlooked data is lost forever. The structure size, the degree of damage, the amount of debris removal, and the complexity of scene reconstruction vary at each site.

We conducted an initial site assessment to determine the scope of the investigation. This included considering if any equipment or extra manpower was needed, determining the structure’s safety, and determining what areas warranted further study. In this case, the site exam took approximately five hours.

Our fire investigator did an initial walkthrough of the entire site and structure. An initial walkthrough to determine the scope of the investigation is integral before moving forward. The point is to assess all areas pertinent to the fire’s origin and spread. Looking at only some areas can result in valuable data being missed.

During the initial scene assessment, we examined this structure, starting from the area with the least amount of fire damage to the area with the most fire damage. We started with a thorough examination of the exterior of the building, which began at the front of the garage. We then worked our way around the house in a clockwise direction.

Figure 3: The front of the garage.

The front of the garage on one gable end sustained only minor fire damage. However, the gable end on the side of the house was burnt through, and the roof was gone.

Figure 4: The fire was in the attic and this portion of the house.

There was no fire damage on the stucco above the side door or the window. This indicates that the fire was in the attic, but it was not venting out of these openings on the main floor and this side of the house.

The gas utility entered on the left-hand side of the house. It was active at the time of the incident and was discontinued during the fire. This meant that at least one gas appliance was to be considered inside the house.

Figure 5: The back of the house.

The roof at the back of the house was mostly consumed. There was more roof remaining on the left versus on the right-hand side. The charring above the two windows on the right-hand side indicated that the fire was venting from the inside out. Thus, the room in this corner of the house (the kitchen) was an area of interest for further examination.

We also observed that the flame venting out of the windows extended into the overhang of the house and made its way into the attic. When they first arrived, the first on-scene firefighters reported fire venting out of these windows.

Figure 6: Another side view of the house.

This gable end of the house was intact compared to the opposite end, consumed mainly by the fire. There is no blackening above the window in figure 6. The hydrometer is on this side of the house, and service was active at the time of the incident.

Figure 7: The front of the house (left) and the trash can from the kitchen (right).

We noticed a considerable number of cigarette butts near the house’s front door. This was an indication that a smoker likely resided in the home. In addition, the fire damage to the top of the doorway and the overhang above indicates that this door was open at some point during the fire.

The front of the door was relatively clean, whereas the back was soot-stained down to the doorknob. This matched the level of staining inside the front room, indicating that the door was closed for most of the fire and only opened in the latter stages. There were signs of forcible entry to the door and its frame. This was determined to result from a passerby kicking in the door to try and look for occupants before fire crews arrived.

After examining the home’s exterior, we found the fire damage consistent with a fire that originated inside the structure.

Figure 8: The living room.

The interior scene exam began at the front door, which opened into the living room. The contents of the living room were mostly unburned. There was blackening to the drywall on the ceiling and walls, and the fire damage increased in severity towards the left-hand side of figure 8. There was charring on the wood door frame on the left.

Smoking materials were observed on the coffee table in the living room. As such, we had to consider that the occupant may have also smoked inside the home. The doorway with the charred frame accesses the hallway behind the wall shown in figure 8.

Figure 9: The hallway.

We observed more heat damage in the hallway than in the living room. The damage was greater at the top portion of the hallway and lessened towards the floor. This fire pattern results from the hot gas and smoke layer travelling along the ceiling.

The charring on the inward swinging door and the drywall behind it indicate that the door was open at the time of the fire. Moreover, the pattern on the door and the walls shows the effects of the fire moving into the hallway from behind where the cameraperson is standing.

Off this hallway were two bedrooms and a bathroom. However, these rooms mostly sustained smoke damage and not much heat damage. This data provided the direction of the fire travel from the entry toward the end of the hallway.

Figure 10: Entry into the kitchen.

The roof was burned away in the kitchen area. The pantry door and the opposite doorway showed a loss of mass at the top, with charring lessening towards the doorknobs. This pattern is a result of the fire travelling out of the kitchen.

We also examined the basement and garage. The basement had a few feet of water from fire suppression activities and did not sustain any heat damage. Neither the electrical panel nor the distribution wiring in the basement was damaged. The gas furnace and hot water tank were also undamaged by the fire, and those were the only two gas appliances within the house.

The attached garage was examined, and the contents were not heat-damaged. The fire patterns observed inside and outside the house indicated that the room of origin for the fire was the kitchen, more specifically, the side of the kitchen shown in figure 11.

Figure 11: The side of the kitchen was identified as the area of origin.

The roof in the kitchen area was mostly consumed by fire. This side of the room was covered in drywall insulation, charred roof trusses, and shingles. Therefore, it had to be excavated before the scene examination.

Removing the debris enabled us to uncover the contents of the kitchen. These contents were then examined to narrow down the area of origin. The upper and lower cabinets along the two walls were consumed mainly by fire.

Figure 12: The left-hand side of the firewall.

Our investigator noted that the metal on the side of the fridge facing the stove displayed greater discoloration and deformity from heat exposure than on the other sides. In addition, there was a base cabinet located between the fridge and the stove. Portions of the bottom and the sides of this wood cabinet remained, and the consumption of the cabinet was from the top down.

There was more significant fire damage on the left side of the stove compared to the right. This data provided a more defined area of origin. Thus, we narrowed the fire origin to the area between the fridge and the right-hand side of the stove.

Figure 13: The reconstruction of the stovetop with the pot hypothesized to be the ignition source.

We reconstructed the items on the stovetop: two ports and an electric grill. The two pots matched the patterns left on the surface of the stovetop and those on the control panel and the side of the electric grill. The food inside the small pot on the rear burner was not burnt, and the plastic handle facing the large pot was consumed.

This pot had an oily residue. The inside and outside of the large pot were discoloured from exposure to high heat. The control panel was damaged by heat exposure from the lower left to upper right. The wood base cabinet between the fridge and stove was consumed.

Additionally, the electric grill was more fire damaged on the left side facing the large pot. At this point in the investigation, the working hypothesis was that a pot of oil was left unattended on the front left element of the stove. As a result, the oil inside the pot reached its ignition temperature and produced an open flame, which spread to the cabinets above.

Had we stopped at this point and declared this an accidental cooking fire, the investigation would have been incomplete. Other experts could have easily refuted our determination because not all the data had been collected and analyzed. We still had to identify all the available ignition sources in the area and determine which started the fire.

Figure 14: Potential ignition sources in the room of ignition, marked by red circles.

Our fire investigator identified and evaluated all reasonable potential ignition sources proximate to the area of origin. The potential heat sources included a duplex receptacle behind the fridge. There was also a branch circuit wiring that ran through the attic space and an electrical outlet to the right of the stove. Other possible ignition sources were the electric grill on the stovetop, the microwave, the toaster, the range, and another electrical outlet behind the base cabinet to the left of the stove.

1. Data analysis, hypotheses, and hypothesis testing

Data must be analyzed to understand its meaning. This enables us to form and test hypotheses based on the evidence. Here, our investigator hypothesized each potential heat source as part of the ignition sequence for the fire.

Analysis of potential ignition sources

The ignition sequence refers to the factors that allowed the ignition source, fuel, and oxidant to react, thereby causing the fire. The oxidant for most fires is the available air in the atmosphere. Therefore, we analyze all potential ignition and available fuel sources in and near that area of origin.

Questions to ask during this process
  • Is the ignition source competent to ignite the fuel?
  • Is the ignition source close enough to the fuel to ignite it?
  • Is there evidence of ignition?
  • Is there a pathway for a fire ignited in the first fuel to ignite the main fuel?

These questions were considered for each potential ignition source in this case, and each source was hypothesized as part of the ignition sequence for the fire.

1. The fridge

The fridge has a compressor motor, electrical components, and a fan motor, meaning some components could fail and produce heat. In addition, insulation and plastic surround the fridge’s mechanical and electrical components, and they could act as the first fuel for the heat source. However, the electrical and mechanical components of the fridge were intact and showed no evidence of ignition.

1. Receptacles and branch circuit wiring

We then looked at the duplex receptacle behind the fridge, where the fridge was plugged in. Although it was exposed to some heat from the fire, the insulation remained on the conductors inside the box. In addition, there were no signs of an electrical event or failure. Therefore, we concluded that this was not the heat source for this fire.

The branch circuit wiring that ran through the attic space fell from the attic when the ceiling failed, from the fire in the kitchen. The copper conductors were intact, and the insulation was consumed from exposure to the heat of the fire after they fell into the room. Therefore, there was no evidence of electrical arcing, and it was not a competent ignition source for this fire.

Figure 15: The receptacle near the stove.

Next, we looked at the receptacle to the right of the stove. The outlet, as well as the connection between the two plugs and the outlet, were considered as possible heat sources. The plastic remained on the conductors inside the box; it didn’t sustain any heat damage. The connection between the conductors and the terminal screws was tight, with no evidence of failure.

Two male cord ends melted into the face of the outlet from heat exposure. However, the plastic face of the outlet was fairly intact. Therefore, this source did not provide heat for the ignition sequence of this fire.

1. The electric grill

The electric grill on the stovetop was not a competent ignition source as it was not plugged in at the time of the fire. In addition, the bottom of the grill was undamaged, which indicates that the element underneath it was not turned on. This means the grill was neither the ignition source nor the first fuel ignited.

2. The microwave

Figure 16: The microwave

The microwave found on the ground to the right of the stove was thermally damaged from exposure to the fire. The plastic turntable that supports the glass plate inside the microwave was not melted, and there was no food inside. If the motor had failed and created heat, there would have been melted plastic inside.

If the controls had provided a heat source for this fire, there would have been more thermal damage inside the control panel. Therefore, there was no evidence that this appliance provided the heat for the ignition of this fire.

1. The toaster

Next, the toaster was examined. The plastic components inside were intact. However, there was greater fire damage on the exterior than inside the toaster. The copper conductors of the cord had mechanically separated when the toaster fell off the counter during the fire. In addition, the toaster was located to the right of the stove and outside the area of origin.

1. Electrical outlets

Figure 17: The electrical outlet, marked by a circle.

We also considered the electrical outlet between the fridge and the stove. The plastic case was intact, as the cabinet protected it at the time of the fire. Moreover, there was nothing plugged into the outlet at the time of the fire. Therefore, it was not a competent heat source to ignite the wood cabinet, and there was no evidence of any failure. The range cord and the outlet it was plugged into were both damaged by fire and did not provide the heat source for this fire. Thus, we had to consider smoking materials as potential ignition sources.

1. Smoking materials

There was evidence that the occupant smoked in the house. As such, we had to consider that a carelessly discarded cigarette in the kitchen may have been a competent ignition source for this fire. However, one discarded cigarette needs more heat energy to transfer to the available fuels in the kitchen long enough to raise those fuels to their ignition temperature.

A whole ashtray dumped into a garbage can in the kitchen could make one cigarette a competent ignition source. We determined that the garbage can was not the area of origin. The plastic bag and some of its contents were still intact.

1. The range

After all these potential heat sources were hypothesized as part of the ignition sequence for the cause of this fire and disproved, we considered the range as the heat source.

Figure 18: The range.

The inside of the oven was undamaged by fire, meaning the fire did not originate inside the oven. However, the left side of the control panel at the back of the stove was damaged from the bottom left to the upper right. The controls on the left displayed greater damage at the face of the panel.

The position of the control knobs for the two left burners could not be confirmed due to the fire damage to the knobs and the metal stems. The stove element control switches had push-and-turn stems, making the likelihood of accidentally turning on the knob minimal. The electrical components at the back of the control switches did not show any evidence of ignition.

The front-left burner area had a protection pattern, which indicated that a large pot was there at the time of the fire. However, the pot was heavily heat-damaged and had an oily residue inside.

Figure 19: The damaged pot.

We hypothesized that a pot of oil had been placed on the front-left burner while it was turned on and unattended. The oil temperature increased to the point of ignition and produced an open flame. This flame produced sufficient heat long enough to ignite the upper cabinets and their contents. The fire extended from this secondary fuel package to the rest of the room and beyond.

We then compared this hypothesis to all the facts and scientific knowledge relevant to this incident. We confirmed that the point of origin was inside the large pot on the left-front burner of the stove. The pot sustained high heat inside, as evidenced by the discoloration, deformity, and burned oily residue.

The Interior of the small pot on the rear left burner was clean, and the food inside was unburnt. If both pots had been exposed to a fire originating elsewhere, the insides should have sustained more similar fire damage. There was no evidence to support an ignition sequence that involved burning material from above and dropping down into the pot or stove.

Research shows that smooth-top stove elements can provide the heat required for cooking oil to reach its ignition temperature. Moreover, the duration and temperature of the heat produced by flaming oil are sufficient to ignite the cabinets and their contents directly above the stove. This confirms that the hot element was a competent ignition source.

The first fuel to ignite was oil. The oil was hot enough and had a pathway to the next available fuel: the cabinet and its contents above the stove. The evidence supports the hypothesis that this fire resulted from an unattended pot of oil on a hot element on the stovetop.


No matter what type of fire or explosion you’re dealing with or how large the loss is, the scientific method must be used. We recognize the need, define the problem, collect the data, analyze the data, develop hypotheses, test those hypotheses, and select a final hypothesis. In cases with liability exposure or subrogation potential, a properly conducted investigation using this methodology will stand up to any challenges as your file moves forward.