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An In-depth Analysis of Fire Investigation: From Process to Mentality

November 11, 2021

Origin and Cause are constantly listening to their clients and customers and one of the “burning”, recurring topics have been fire investigation and its process. In this article, we will discuss the methodology of fire investigation and how a fire investigator comes to the conclusion of the origin and cause of a fire whilst investigating scenes such as the following:

The above photos depict a basement that caught fire. The tenants in the home were without heat and were using propane heaters to derive warmth. Unfortunately, combustibles were placed too close to the heating equipment on the propane tank, causing a fire.

These photos depict the result of a bedroom fire in a three-story apartment building where the entire room was engulfed in flames. After going through the contents and evidence, the cause of the fire was determined to be “careless smoking”.

Here’s how the charred mess above can shed light on the origin and cause. We dissect exactly what occurred to draw such conclusions before filing the final investigation report. The guidelines discussed are found in the NFPA 921 Guide for Fire and Explosion Investigation as it sets the bar for science-backed investigation and analysis of fire-based incidents. The publication is also accepted as authoritative in the eyes of the Canadian court as it helps portray a non-biased, systematic approach to the investigation.

The Scientific Method

The systematic methodology employed in the physical sciences, including fire investigation, is based on the scientific process, and the investigator can only establish and defend their origin and cause determination by consistently applying the scientific method to every investigation.

The Scientific Method Entails the Following Steps:

Identifying the problem: First, acknowledge the need and understand that there is a problem, such as a fire or an explosion, to comprehend what happened.

Defining the problem: An origin and cause investigation is required to determine where and how the fire or explosion started.

Data collection: Gathering and documenting all the facts about the incident. Some of the steps during this process include:

  • Occupant and witness interviews
  • Scene examination
  • Evidence collection and recognition
  • Site diagrams
  • Photography
  • Research

Data analysis: Attribute meaning and comprehend the data that has been collected.

Developing a hypothesis: Generate explanations for the occurrence based on the evidence that has been gathered.

Testing out the hypothesis: Compare each hypothesis to all of the facts that are known. The testing is intended to reject rather than verify the hypotheses. This approach helps avoid relying on data that solely supports the hypothesis.

Select the final hypothesis: Select the final hypothesis after reviewing the entire procedure to ensure that all credible data has been accounted for and all alternative hypotheses have been evaluated and dismissed.

Residential Structure Case Study

(Structure before the fire as seen on Google Street View: A single-story, single-family residence with a single attached garage)

(structure after the fire incident)

Step 1: Examining the Scene

The primary goal of a scene examination is to gather all relevant information and document the scene. Any data that was overlooked will be lost forever when this structure is pulled down or repaired. The size of the structure, the degree of destruction, the quantity of excavation or debris removal, and the difficulty of restoration varies at each loss site, therefore this particular site examination took about five hours.

At this stage, investigators tend to pre-plan as that will help make the overall investigation smoother. They consider factors such as:

  • Safety concerns
  • The requirement of specialized tools
  • Assessing the scene to determine if other engineering experts will be required
  • Security scenarios for the site (if 24-hour security surveillance will be required)
  • Giving reasonable notice to all interested parties and offering them an opportunity to take part in the investigation (this is to prevent the other party from claiming evidence spoliation later)

These factors were considered for this case. The site was assessed both inside and out, along with the areas that warranted further study. This helped determine the overall scope of the investigation moving forward. The aim is to analyze all places that are relevant to not just the fire’s origin but also its spread, and by failing to do so, significant data can be missed.

During the initial evaluation, data is collected simultaneously to help determine the origin of the fire. This investigation started by examining the area with the least amount of fire damage and moved to the area with the most amount of fire damage. The assessment revealed that the front of the garage sustained only minor fire damage.

(side of the house)

Moving to the side of the house, the assessment revealed that the gable was burned through as the fire was in the attic of the house. With the side window and stucco remaining unharmed, it could be inferred that the fire was not venting out of these openings of the house. The initial examination also showed that the gas utility on this side of the house was disconnected during the fire.

(back of the house)

The back of the house revealed that the roof had been consumed but the left side had more roof intact. The charring above the right-hand side windows helped determine that the fire was venting out from these openings, from the inside. We narrowed in on the room on the right-hand side of the house for further examination.

(gable end of the house)

The gable end of the house on the opposite side was mostly intact with no blackening above the window.

(front of the house)

The front of the house revealed various cigarette butts, pointing to at least one smoker in the residence. The fire damage to the doorway indicates that the door was open during the latter stages of the fire, by studying the amount of staining present on the door and the doorknob.

The external scene examination corroborated the theory that the fire had originated inside the house and extended out.

Step 2: Data Collection

The front door opened into the living room. The contents of the room were mostly unburnt, with fire damage severity increasing towards the left side. Smoking materials were found on the coffee table, pointing out the possibility of the resident smoking inside the house.

The heat damage spread across the hallway with more damage towards the top layer of the hallway than the floor, which is consistent with a smoke and gas layer traveling along with the ceiling. The damage to the rooms connected to the hallway was mostly smoke damage and not heat damage which helped determine that the direction of the fire was towards the end of the hallway.

The burn pattern on the pantry doorway and the door opposite is consistent with the theory that the fire traveled out of the kitchen. The basement and the garage had not sustained any heat damage. The roof of the garage however had burned away due to the fire traveling through the attic above the kitchen.

After examining both the inside and outside of the house, it seemed most likely that the room of origin was the kitchen.

The roof in this area of the kitchen was mostly consumed by the fire. This room had to be excavated and the debris was removed layer by layer to uncover and examine the contents of the room, to narrow down the area of origin.

The metal on the fridge facing the stove had a greater degree of deformity and discoloration from exposure to the heat. The left side of the stove had greater damage than the right, which helped pinpoint that the area of origin was the area between the fridge and the right side of the stovetop.

After reconstructing and studying the contents on top of the stove, a working hypothesis was established that the origin of the fire was inside the large pot. A pot of oil was left unattended on the left element of the stove, causing the oil to reach its ignition point. This caused a fire that spread to the cabinets above.

However, a firm conclusion ruling it an accidental cooking fire could not be drawn yet. More data had to be analyzed to make the claim irrefutable. All reasonable and potential ignition sources, which were close to the origin point, were identified and evaluated.

The Potential Heat Sources that could have Started the Fire are:

  • The refrigerator
  • Duplex receptacle behind the fridge
  • Electrical outlet to the right of the stove
  • Branch circuit wiring running through the attic space
  • The electric grill on the stovetop
  • The microwave
  • The toaster
  • The stove range
  • The electrical outlet behind the base of the cabinet to the left of the stove

Now that the data has been collected and documented, it must be analyzed, to derive the accurate meaning behind it. This data and evidence will help form various hypotheses. The investigator has to hypothesize each potential heat source as part of the ignition sequence for the fire. The ignition sequence is a sequence of factors that allowed the ignition source, fuel, and oxidant to react, causing the fire.

Step 3: Analyze / Develop / Test

To determine the ignition sequence, there must be a thorough analysis of all potential ignition sources and available fuel sources in and around the area of origin.

The investigator must ask the following questions:

  • Competency of ignition source: Does the source have sufficient energy and is it capable of transferring that energy to the nearby fuel source long enough to raise the temperature to the ignition point?
  • The proximity of ignition source: Is the ignition source close enough to the fuel to be capable of igniting it?
  • Availability of evidence of ignition
  • Pathway for a fire ignited in the first fuel leading to the ignition of the main fuel

Analyzing the Different Ignition Sources:

All ignition sources were considered, keeping the above questions in mind.

  • The Refrigerator

The fridge has electrical components, a compressor motor, and a fan motor: components that could fail and produce heat. The insulation and plastics surround the electrical and mechanical components of the fridge and could be the first fuel for the heat source, however in this case the electrical and mechanical components were intact, showing no signs of ignition.

  • The Duplex Receptacle

The duplex receptacle behind the fridge where the fridge was plugged in was exposed to some heat from the fire, however, the insulation remained intact on the conductors and it showed no signs of any electrical failure hence it could not be the heat source for this fire.

  • The Branch Circuit Wiring

The wire had fallen from the attic when the ceiling failed due to the fire. The copper conductors however were intact, even though they showed signs of heat damage. It was not a competent ignition source because there was no evidence of electrical arcing.

  • The Receptacle Next to the Stove

The outlet and the connection between the plugs and the outlet seemed like a potential ignition source. The plastic on the conductors did not sustain heat damage. The connection between the terminal screw and the conductor did not show signs of failure. Two male cord ends had melted due to exposure to the heat from the fire. When the microwave and toaster tumbled to the floor during the fire, the stranded copper conductors linked to these two male ends broke away from the cords leading to them. This source did not provide the heat for the ignition.

  • The Electric Grill

The electric grill on the stovetop was not the source of heat as it was not plugged in at the time of the incident. The bottom of the grill was undamaged as well, suggesting the element at the bottom was not turned on. The grill was neither the first fuel ignited nor the ignition source.

  • The Microwave

The microwave was on the right side of the stove and had fallen to the ground. It showed signs of thermal damage due to fire exposure. The plastic turntable inside had not melted, proving the motor had not failed. If the controls had provided the heat, the wires inside would have melted but the wires had the plastic intact on them. There was no evidence to prove that this was the source of ignition.

  • The Toaster

The toaster was examined for damage and the exterior showed signs of greater damage compared to the interior. The toaster was located to the right of the stove and not in the area of suspected origin.

  • The Electrical Outlet Between the Fridge and Stove

The plastic face of this outlet was intact as it was protected by the cabinet at the time of the fire and nothing was plugged into this outlet at the time. It did not show evidence of failure and was not the heat source that set the wood cabinet on fire.

  • The Smoking Materials

The smoking materials had to be considered as the potential ignition source as well. It was possible the resident could have carelessly discarded the cigarette. A single cigarette couldn’t be the potential heat source but if the entire ashtray was dumped into the garbage it could make the cigarette a competent heat source. The garbage can was not present in the area of origin and the contents of the can be intact to some extent.

  • The Range

As all of the above sources were hypothesized as potential ignition sources and disproved, the range was considered as the heat source. The inside of the oven was intact so the fire did not originate inside the oven. The left side of the control panel located at the back of the stove showed signs of damage from the bottom left to the upper right.

This pattern was a result of the lower cabinet on the left burning and the control panel being exposed to the heat. The control panel on the left showed signs of greater damage on the face of the panel when compared to the inside of the panel.

The stove element controls were the push and turn type so the chances of the knob being turned on accidentally were minimal. Due to the extensive fire damage, the position of the left knobs could not be determined. The front left burner showed signs of a protection pattern indicating a large pot was present at the time of the fire. The pot had damage on the inside and outside with oily residue on the inside.

Step 4: Test and Select the Final Hypothesis

After testing and disproving all of the above hypotheses, the investigator came back to the hypothesis that a pot of oil was left unattended on the left burner. The oil temperature increased gradually to a point where it reached its ignition temperature causing an open flame. This flame sustained for a long duration, causing the cabinets to ignite. The fire then spread from the secondary fuel package to the rest of the room.

This hypothesis must be compared to all the other hypotheses along with all the known facts and scientific knowledge pertaining to this incident.

It was now evident that the fire started due to an unattended pot, situated on the left side of the stove, which caught fire. The small pot on the rear left burner was clean and the food was unburnt. If both pots had been exposed to a fire having started elsewhere, they would have shown similar signs of exposure and damage. There was no evidence to suggest an ignition sequence involving something falling from above into the pot to cause a fire. Scientific research also shows that smooth top stoves can provide enough heat for the oil to reach its ignition temperature. The duration of the heat caused by the ignition of the oil is sufficient to set the cabinets ablaze.

This helped establish the following:

  • The hot stove element was a competent ignition source
  • The oil was the first fuel to ignite
  • The oil was hot enough and had a pathway to the next fuel source which was the cabinets

The evidence supports the hypothesis that this fire was due to a pot of oil left unattended on a hot stove.


Although this was a non-complex fire, the methodology that has to be applied in every case is the scientific method irrespective of the scope of the explosion or the simplicity of the incident. These are the steps that are applied to understand the origin and cause of a fire and explosion. This methodology will also stand up in cases where there is a liability or potential subrogation. No matter how complex the scene of the incident, a sound and scientifically conducted investigation will always hold its ground in any court of law or cross-examination.