The cause should be considered as a combination of circumstances that result in the fire. Establishing the cause will include looking for potential ignition sources, a means that would explain how the fire started and developed, and any acts or omissions that may have contributed to this. The evidence should demonstrate that all the ingredients were present and an explanation offered for how they relate to each other. For example, the presence of an ignition source such as matches is not proof they were responsible for a fire.
The fire investigator is most likely to encounter the HSE where they have an interest in a fire related to a potential breach of health and safety requirements in a workplace at a fire attended by fire and rescue services.
Initial assessment
Before any activity begins, an early assessment is essential to consider the level of investigation required and the associated level of proof needed to establish the cause. In addition, thought should be given to who else may have an interest and who has primacy for the scene. Where there are indications that a fire is suspicious at the start of the investigation, or at any point during the investigation, this information should be passed to the police and the appropriate processes followed.
Scientific method
Using the scientific method is generally recommended as the approach to investigating the cause of a fire. With this method, the investigation findings can be presented logically and objectively, including stating any gaps or uncertainty in the evidence. This is described in various ways but the core elements are:
- Observing an event, for example, the fire scene
- Defining the problem – the cause of the fire
- Collecting data and evidence – physical and verbal
- Formulating a hypothesis – what does the evidence suggest to be the most likely cause of the fire, including ignition source, materials involved and any human factors?
- Testing the hypothesis and revising if necessary – is the evidence available all consistent with the hypothesis and does it allow other possible causes to be discounted?
- Selecting a final hypothesis – set out the believed cause and supporting evidence, and any limitations or inconsistencies
This is a scalable approach and the amount of time and resources committed to an individual incident will be in line with the expected purpose of the investigation of the cause.
Standard of proof
For most incidents where the only requirement is to complete the Incident Recording System (IRS), it is sufficient for the ‘most likely’ cause to be assessed. This will not usually require detailed notes or supporting evidence to be collated, although it may be good practice to do so. It should be noted that while the IRS does not explicitly require more than an assessment of the cause, there is benefit in achieving the greatest degree of confidence possible about the cause of all fires, as the subsequent data will directly inform service activity through analysis and intelligence work. As such, consideration should be given to ensuring clear guidance on the expectations of crews and the standard of origin and cause investigation required to support the ability of the fire and rescue service to target resources effectively.
Where the cause of a fire is being sought as part of a formal investigation or another agency’s investigation, the standard of proof required should be confirmed at the outset. Most commonly, this is where the evidence must be beyond reasonable doubt, as employed in criminal cases and some coroners’ inquests, or based on the balance of probability (more likely to be true than not) for civil cases and most coroners’ inquests.
Locating the origin of the fire
The first requirement in establishing the cause will usually be to identify an area of interest (or radius of error as it is sometimes known) that is believed to contain the origin of the fire. The area of interest will be larger than the believed actual point of origin, to allow for the discovery of associated evidence and, as its name suggests, some scope for error in the initial assessment.
In most cases, the affected area of fire damage is relatively small and so the possible seat of the fire may be fairly obvious and localised. However, where the fire has affected a greater area, a logical method should be adopted to assist in narrowing down the scene to a specific area of interest. For example, the initial assessment might involve a walk around the scene to observe the damage.
This stage should not involve the disturbance of material. Generally, the process adopted will be to conduct an external and then internal viewing, noting the effects of the fire or other salient features such as signs of forced entry or possible evidence. This can also be a good opportunity to formulate a risk assessment.
Considerations for establishing an area of interest
There are a number of ways in which the investigator can identify the area of interest. These should be used to provide information that is consistent in identifying where the fire started and how it spread. If not, the evidence may have been moved at some point during the fire or firefighting, or it may have been misinterpreted. Either way, an explanation should be sought for any inconsistency or a different hypothesis considered.
‘Post-fire indicators’ is the general term used to describe the different clues or effects the fire leaves behind on a structure or contents within. This can relate to damage caused by direct burning (flame) heat, smoke or a combination of these. These clues can help the investigator identify the potential origin of the fire, its development and the location of items at the scene.
Another approach is to identify the lowest and most severe area of burning as this will typically indicate the point of origin. However, this will not always be the case and the context of the fire should be considered. For example, one area may have burnt for longer because crews were unable to reach and extinguish it, or there was a higher fuel loading in one location or liquid or dropping material/embers may have spread the fire to a lower level.
Witness evidence can also be helpful in confirming the origin (and cause) of a fire but the investigator should always ensure that the physical evidence matches the verbal information as it is sometimes possible for witnesses to be deliberately or unintentionally misleading.
When assessing the post-fire indicators, time should be taken to ensure any activity that might have affected the scene before the investigation is known. This can include actions by members of the public, the fire and rescue service and other first responders. The choice and application of various firefighting techniques may also influence or alter the expected post-fire indicators.
Excavating the area of interest
Once the area of interest has been established, the next step is to try to identify the cause of the fire. This should include identifying any potential sources of ignition, the materials or means by which a fire took hold and developed, and the mechanism by which it happened.
Excavating an area can be a time-consuming process and the degree of care needed will need to be assessed regarding the level of proof required, the resources available and the type of evidence being sought. For example, retrieving small or fragile items intact will require greater care in excavation than a large, solid item. The way in which items are handled, retrieved and preserved will also be determined by the nature of the investigation.
It is important to be aware of items that are unusual or out of context (for example, a can of petrol may be expected in a shed but is less likely in a lounge). What is not present can also sometimes be of value and should be noted.
There are several readily available lists of typical causes of fire but the investigator must always follow the evidence and be mindful that new types of fire are regularly found and that knowledge of fire constantly changes and expands.
Checking the hypothesis
Once all the evidence has been identified and recorded (if appropriate), the investigator should review it to ensure it is consistent with the identified cause. If not, the information should be checked and further investigation undertaken or a new hypothesis developed, as necessary.
One way to check the physical evidence is by recreating the scene. This can be useful for larger fires. It allows the room to be cleared of all items, and then the major or important items put back in the location in which they were believed to have been during the fire. From this the investigator can check the post-fire indicators against the structure, that moveable items correlate, and that the fire development can be explained by the evidence.
Resources and tools
There are a number of resources and tools that the investigator may find of use. These include using dogs or bespoke equipment that can help to indicate the presence of possible accelerants.
In addition, a range of small tools and personal protective equipment (PPE) will be required and, for larger incidents, specialist access, lighting and other equipment may also be necessary.
Reference sources
A range of good reference sources is available to those required to establish the cause of a fire. These include specialist fire investigation publications covering peer-reviewed books and articles, online sources and communities.
It is also useful to stay up- to-date with related areas such as new products, construction materials/designs and human behaviour, and to understand how these could lead to fires.