Fire & Explosion Investigation: Short Guide Into the Procedure

Article by: Cameron Gallacher

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Investigating fire and explosion incidents is a complex and time-consuming process. Fire investigators (FIs) come to the scene to find many kinds of incidents, ranging from stove fires to multiple fatalities. The investigators must be meticulous in performing their duties in the dispatched location. They have to be specific in documenting every procedure, assessment, evidence collection, interview, and follow-up.

Fire investigators must be thorough in documentation; the information must be factual and grounded on solid evidence. The investigators cannot reach a definitive conclusion for the event without any evidence. Therefore, fire companies should not do extreme overhauls on fire scenes to prevent obliterating fire patterns. Also, firefighters have to remain patient as FIs perform their duties because FIs have limited time, resources, and people.

FIs not only investigate arson cases. Their duties have expanded to inspecting fires and explosions in their specific districts. Their fire service may also include:

  • Preventing accidental fire.
  • Catching criminals.
  • Implementing fire code changes.
  • Identifying potentially hazardous products that need to be recalled.

Fire investigators and officers have to create an entire fire report providing information about the facts of the case. What is the fire origin or fire cause? Were there smoke alarms at the scene, and did they operate as the fire started? Did the occupants hear the alarms? If not, what was the cause? These questions provide critical information to the fire administration or fire prevention bureaus. The city can use the information to determine the proper action to prevent future fires.

Fundamental Fire Investigation Terminologies

It is crucial to understand the fundamental terminologies in fire investigation to understand its complexities. By knowing the essential terms, you can easily unlock the process of fire and explosions investigation. Here are the basic terms:

Fire and Explosion

Fire is created through rapid oxidation, which produces varying heat, light, and sound levels. Fire’s origin has three components: fuel, oxygen, and the stimulus. The stimulus can either be in the form of an electrostatic discharge, shock, heat, impact, or friction. When fuel and oxygen mix proportionately, adding any stimulus to the mix can start the fire.

An explosion is sudden and forceful energy expansion that creates pressure differentiations, shock waves, seismic effects, thermal effects, and material propulsion into the immediate environment. The intensity of the explosion varies depending on what kind of explosive material was used, how much explosive material was ignited, the humidity and temperature at the time of the blast, and other factors.


The definition of arson varies depending on the source, e.g., the Federal Bureau of Investigation. For fire and explosion investigations, we follow the description given by the National Fire Protection Association in their NFPA 921 Guide for Fire and Explosion Investigations. Based on the guide, arson is “the crime of maliciously and intentionally, or recklessly, starting a fire or causing an explosion.”

NFPA 921

The NFPA 921 Guide for Fire and Explosion Investigations is a document that serves as a guide for fire investigations. The guide was written primarily by FIs, engineers, scientists, and legal partners. Although it is not a requirement for investigating fires or explosions, it is highly recommended because it is most likely used as a reference in court. Sections of the document include information on collecting evidence and analyzing fire patterns, electrical fires, marine fires, and others.

Evidence and Determination

Evidence refers to tangible artifacts procured from the fire scene and the non-tangible information taken from interviews and interrogations. All evidence is grounded on facts and not on opinions. The facts disclose a narrative that can lead to the origin and cause of the fire or explosion. Evidence is the key to solid determination; without it, there is no proof of the basis for the findings.

Determination refers to the FI’s finding based on evidence uncovered during the investigation. It cannot be conclusive if the result is not based on science and solid evidence. Determination has four distinct categories: Accidental, Incendiary, Natural, and Undetermined.

Accidental fires do not involve deliberate human action, as is the case for most electrical fires, whereas incendiary fires are intentional. Natural fires are caused by natural causes such as extreme summer heat and lightning strikes. Undetermined fires mean that the FI came up with two or more hypotheses but cannot decide on a definitive finding because of the lack of evidence.

Origin and Cause

The origin refers to where the fire started. During fire investigations, FIs rely on fire patterns to track the source of the fire. The fire investigator may also use electronic data, witness information, arc mapping, fire dynamics, and other evidence to determine the origin. Sometimes, the incident origin is not easy to narrow down to a specific location. If severe damage to a room and the exact location cannot be pinpointed, the room can be considered the “area of origin.” If there are multiple origins, it may lead FIs to speculate that the fire is incendiary.

The cause refers to how the material and ignition source came together to form the fire or explosion. FIs also try to determine the fuel used and the ignition source. When collecting data, FIs attempt to determine if electrical issues caused the fire, ignitable liquids, or any other fuel or heat source. Knowing the cause of the fire can lead FIs to determine if the fire was incendiary, accidental, or otherwise.

Interview and Interrogation

The interview is a face-to-face conversation for information gathering. It can be requested by the FI, victim, suspect, witness, or attorney. The purpose of the interview is to gather pertinent facts for the case and not accuse anybody. The discussion can transition into an interrogation. However, the FI should not use accusatory language unless the interviewee is suspect.

An interview can become an interrogation when a person is considered to have a connection to the cause of the fire. Although the interrogation is similar to an interview, the questioning may become accusatory. The FI uses psychological communication techniques to get as much needed information or a total confession.

The Scientific Method

The best way to establish the facts of a case is through a scientific-based investigation. The scientific method uses a structured and systematic approach to investigating fires and explosions. There are seven fundamental steps, and each one of them is crucial. Having any misunderstanding at any stage can result in an erroneous conclusion, leading to the arrest and conviction of innocent individuals.

Step 1: Need Recognition

The first step in the scientific method is to determine the need for the investigation. It can identify and arrest the person responsible for starting the fire or explosion or preventing subsequent fires. If the information on one fire is gathered and analyzed accurately, it can avoid or investigate future fires.

Step 2: Problem Identification

When fires and explosions occur, there is a big chance of injuries or casualties. Once an investigation starts, it may reveal other possible dangers that have not been initially identified. There could be electricity dangers, hidden bodies, or imminent collapse of affected structures. Investigating fires aid in fire prevention, which helps lessen the chance of firefighter injury, property damage, and environmental pollution.

Step 3: Data Collection

Although any investigation aims to prevent fires in the future, collecting data aims to determine the origin of the fire and its cause. If the fire is incendiary, gathering the correct data can lead to finding the perpetrator of the crime.

The data can come in the form of tangible artifacts. Everything present in the scene can provide evidence. However, items missing from the fire scene may also lead to clues. Are there missing equipment, pictures, inventory items, and luxury items? FIs need to have high data collection literacy to ensure they do not waste too much time and effort gathering unimportant information.

Data can also be intangible, such as the statements made by witnesses, victims, and suspects or the observations of the FI and the fire crew. Hence, it is crucial to learn what data to collect and disregard. Knowing which data is relevant and what collection method to use comes with proper training, education, and experience.

Step 4: Data Analysis

Data analysis is a crucial function that requires the FI’s attention to detail. The FI has to look for specific patterns and cues in the collected data to formulate a definitive hypothesis. The evidence collected should come from the fire scene and is relevant to the case. The investigator must learn to connect the proof to develop a credible finding.

There are many kinds of questions involved in the analytical process. These questions must be answered to fit the pieces of the puzzle together. When analyzing the data collection, an investigator must be impartial and unbiased. If the FI has already predetermined a cause without collecting data and evidence, it will lead to an erroneous determination. An investigator must always avoid confirmation bias because it can lead to the arrest and conviction of innocent people.

Note that bias can lead to inaccuracies and imminent downfall. Once investigators are discredited as expert witnesses, they lose their credibility and careers. Hence, it is essential to be as fair and unbiased as possible.

Step 5: Formulation of the Hypothesis

Formulating a hypothesis is complex and challenging. Many times, evidence gets destroyed or becomes difficult to discern. Although the evidence may remain at the scene, the fire’s damage or the explosion may make it hard or impossible to collect. In some cases, there are no witnesses or at least security camera footage to serve as data sources.

Fire and explosion investigations are fundamentally theory-building. Investigators have to outline a narrative of events before, during, and after fire and explosion incidents. In many instances, it requires an inductive process to formulate a hypothesis. The inductive reasoning process requires a great deal of education, training, and field experience to master it. When an investigator formulates a hypothesis, it should be backed by physical evidence, witness statements, observations, and experiments, primarily when used in court.

Step 6: Testing the Hypothesis

Testing the hypothesis requires a deductive process. FIs use their training, education, guides, references, research, standards, experiences, and experiments to put the hypothesis to the test. The hypothesis has to be thoroughly examined and pass scrutiny. It must be discarded if it cannot stand up to critical analysis.

The investigator’s duty is not to prove the hypothesis but to check if it stands up to every test. It is probable to stand up to any test applied to it. Nonetheless, it has to be peer-reviewed to check for confirmation bias. Also, investigators have to consider other hypotheses and document their findings. There are no standardized tests for all kinds of fires and explosions. Therefore, it is vital to use many tests to ensure correct results.

Step 7: Conclusion

Before making any conclusions definitive and using them in court or for other purposes, FIs need to perform a peer review of the study. Doing this can eliminate confirmation bias and make the determination correct. There may be things that other investigators or experts may see that the FI has not seen. 

It is of utmost importance that investigators present their findings as understandable and straightforward as possible. Not everybody in court is an expert in fire investigation lingo. The presentation should be as clear and as intelligible even to a novice. It should include all the facts of the case, a firm conclusion, and a final determination. The determination should be based on factual evidence, supporting hypotheses, and findings.

As long as the first six steps are appropriately implemented, choosing the correct hypothesis should not be an issue. If an investigator cannot decide between two or more hypotheses, there may be an error in any of the previous steps. The error can be in the collecting of data, the analysis, or formulating a hypothesis. If not, it could be because there is insufficient data or another hypothesis that has not been considered.

Bottom Line

It is a good recommendation to follow strict rules when performing fire and explosion investigations. Adhering to protocols, procedures, and policies will make a fire investigator a credible expert when the findings get into court proceedings, depositions, or insurance investigations. When investigators provide detailed and accurate data in their reports, they will benefit everyone involved.

A fire investigation is credible only when it uses the scientific method. It should adhere to the National Fire Protection Association (NFPA) 921 Guide for Fire and Explosion Investigations. Only then can the court of law, insurance companies, and other people involved have peace of mind that the investigation was based on high moral standards and guidelines.

If you require Fire & Explosion Investigation services, our experts have the technical knowledge combined with years of experience to analyze any complex incident. Our investigations are conducted based on the NFPA 921 and sound engineering practices. To learn more about us and our services, check out our website.