Fire Investigation

Fire investigation involves the examination of all fire-related incidents once firefighters have extinguished the fire. The practice is similar to the examination of crime scenes in that the scene must be preserved and evidence collected and analysed, but with numerous additional difficulties and dangers. The investigation will include closely surveying the damaged scene to establish the origin of the fire and eventually establishing the cause.

However in order to effectively examine and evaluate a fire scene, it is imperative that the investigator has a detailed knowledge of the chemistry and behaviour of fire and its effects.

Nature & Chemistry of Fire
Fire occurs due to the exothermic reaction of combustion (burning), producing heat and light. In order for a fire to occur, three vital components must be present: a fuel source, an oxidant (O2) and a sufficient amount of energy in the form of heat. Together these make up the fire triangle. A fourth factor can also be described – a self-sustaining chemical chain reaction – to produce the fire tetrahedron. The absence of any of these conditions will result in a fire not starting or extinguishing through smothering (oxygen removal), cooling (heat removal) or starving (fuel removal).

Solid and liquid materials do not actually combust, but the process of heating causes them to produce vapours which can burn. This is the process of pyrolysis. Through this pyrolysis products will be formed, flammable and volatile substances of low molecular weight caused through the decomposition of materials by fire.

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The colour of flames can vary depending on the materials involved in the combustion. The colour of a flame is basically determined by the wavelength of light emitted, which varies depending on the material. For example, red/yellow/orange flames are commonly encountered when carbon is present. Inorganic substances can produce more obvious colour differences, such as copper which will cause a green flame.

Heat produced by a fire can spread in one of three ways; convection, conduction and radiation. Convection is the transfer of heat through air circulation, and only occurs in liquids and gases. An example of convection is the heat from a fire rising and heating the ceiling of a room. Conduction is the transfer of heat through a medium by direct contact, such as a fire heating a metal beam which transfers the heat elsewhere. Radiation is the emission of heat as infrared radiation without a medium, such as a fire heating and igniting a nearby sofa.

Ignition will occur when all required conditions to start a fire occur, producing either a smouldering or flaming fire. This will often be induced by the addition of heat to a fuel in air, which can be caused by various sources such as exothermic chemical reactions, friction, solar radiation and electricity.

The temperature required for ignition to occur varies depending on the fuel. The flash point is the minimum temperature at which fuel favour is momentarily ignited in air by an external ignition source. However this will not necessarily sustain combustion and produce a fire. The flame or fire point is the minimum temperature at which enough vapour is produced to allow continued combustion. This is usually a few degrees higher than the flash point. Both the flash and flame point of a substance can be determined by placing a small amount of sample in an airtight container, gradually increasing its temperature whilst periodically adding an ignition source, and then measuring the point at which the flash and flame point is reached.

The spontaneous ignition temperature, also known as the auto-ignition point, is the lowest temperature at which a substance will ignite without any external ignition source. This is measured by heating a sample, studying the central temperature of the material and documenting the temperature at which ignition spontaneously occurs.

The flash point, flame point and spontaneous ignition temperature are the lowest temperatures at which a material has ignited when heated experimentally, though these actual temperatures can vary and so should only be used as a guideline. Different fuels also have individual lower and upper flammability limits, the lowest and highest concentrations of flammable gas required for combustion. If the concentration falls outside of this flammability range, combustion will not generally occur. Substances such as hydrogen have wide flammability ranges, making them particularly dangerous.

Not all types of fire produce flames. Smouldering is a form of flameless combustion which occurs at the surface of the material in cellulosic substances that can form a solid char. The presence of a smouldering fire is characterised by extremely localised burning and the production of thick, tarry smoke. The surface temperature can be linked to the colour of the smouldering. For example, dark red surfaces suggest a temperature of 500-600oC, whereas a white surface indicates temperatures in excess of 1400oC. The rate of propagation is dependent on the material burning and the amount of oxygen available. Only low concentrations of oxygen are required for smouldering combustion, but if sufficient oxygen is supplied, smouldering fires can then produce flames. Cigarettes are a common cause of smouldering fires when left in contact with upholstered furniture, for example.

Spontaneous Combustion
Spontaneous combustion refers to the sudden ignition of a material without an external ignition source such as a flame or spark. The phenomenon occurs as a result of exothermic chemical reactions occurring within the material, releasing heat. In cases where the material is piled together, the heat cannot dissipate effectively and so the temperature within the material rises. The rise in temperature causes chemical reactions to accelerate, producing even more heat. The temperature can rise until the flame point of the material is reached, causing ignition. Spontaneous combustion tends to be characterised by the apparent source of the fire being the centre of the material, as heat is dissipated more readily from the surface, thus resulting in the centre reaching the highest temperature. Rags soaked with oil, sawdust or piles of hay have been known to spontaneously combust.

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Fire Scene Investigation
The primary purposes of a fire investigation is to establish the origin (seat) of the fire, determine the likely cause, and thus conclude whether the incident was accidental, natural or deliberate. It is vital to establish the cause to ensure similar events do not occur (in the case of natural or accidental) or to allow a legal investigation to be conducted (in the case of deliberate fires).

The initial concern with regards to a fire incident scene is safety. Such a scene has an increased risk factor with possible hazards including heated materials, structural collapse, damaged electricity and gas mains, debris, asbestos, dangerous combustion products and other toxic substances. A dynamic risk assessment should be conducted, the scene must be declared safe and all individuals entering the scene should wear appropriate protective clothing such as hardhats, fire-resistant overalls, steel-capped boots, thick gloves and, in some cases, a face mask. Supplies of gas and electricity should be switched off before the investigation begins.

Information regarding a fire can be obtained from witnesses. Witnesses may be able to provide details of the premises prior to the fire in addition to details of the fire itself, such as suspicious activity or apparent fire spread and smoke colour. Onlookers may even have taken photographs or video recordings of the incident on their mobile phones or cameras. The owner of the building/area may be able to detail the contents and layout of the building as well as any other potentially pertinent facts. However it should always be taken into consideration that civilian witnesses may be unreliable and could even have been involved in the fire incident. Emergency service personnel, such as police and fire fighters, are considerably more reliable as witnesses. Fire fighters in particular may be able to provide useful information on the possible origin of the fire and any unusual conditions. Fire fighters should also be interviewed to identify any disturbances made to the scene during fire-fighting efforts.

Ideally eyewitnesses should be interviewed by an objective individual with experience in interviewing in such a way that the information they provide is not influenced.

Scene Examination
A fire incident should be treated as a crime scene in that the area should be strictly controlled by a cordon to preserve evidence and allow access to authorised personnel only, with the scene and evidence being fully documented. A plan of the premises should be produced where possible to include the locations of objects, though it must be taken into consideration that disturbance may have been caused during fire-fighting efforts.

The investigation should ideally begin with an external examination of the scene. This allows for the identification of entry point, signs of forced entry, indications as to the origin and cause of the fire, artefacts, and any possible safety concerns. All doors and windows should be examined to establish whether or not they were locked during the fire. Once again, fire-fighters may have forcefully entered the building or smashed windows to provide ventilation, and damage caused by the fire itself may appear similar to signs of forced entry. The external examination will also allow for the search for items relevant to the incident, such as tools used to break into the building, ladders or containers of flammable substances. It may also be important to note weather conditions, as temperature and wind conditions can affect a fire in terms of fire propagation and direction.

The interior examination of the scene is then conducted, usually with the production of the layout of the scene detailing the location of items and any bodies. The investigator will generally begin with the area of least damage, allowing investigators to backtrack to the seat of the fire, which will typically be found in a more damaged region.

Establishing the Origin
A vital aspect of the forensic fire investigation is to establish the point of origin of the fire, also known as the seat of fire. There are numerous indicators that can be used to determine the possible origin. The region in which a fire started will generally burn for a longer amount of time, thus will be an area with the worst damage. Fires tend to burn upwards, therefore the seat of the fire is likely to be found at a lower point of burn damage. However this is not always reliable as fires can spread downwards, particularly in the presence of certain fuel sources.

Fire effects on certain materials can indicate the direction of fire. As fire burns upwards and outwards, V-shaped smoke/burn patterns may be found on surfaces adjacent to the fire, with the end of the V pointing towards the point of ignition. However ventilation can affect the path or shape of V-shaped patterns. Smoke deposits of object surfaces can suggest the direction from which the fire originated, and glass and plastics tend to melt in the direction of fire, thus distortion of such materials can act as directional indicators.

Structural damage to the building can also be used to locate the seat of the fire. In some instances buildings may collapse in such a way that the area first weakened by the fire is clear, suggesting this is where fire damage first occurred and thus is the origin. Similarly, windows and ceiling structures are likely to fail in areas close to the seat of the fire first. However this is by no means an accurate method of locating the seat of the fire, as the collapse and damage of a building is affected by numerous factors, not just the fire itself.

It may be possible to determine the area in which a fire began based on the operation of smoke and fire alarms. There may be some form of record of which alarm was triggered first, suggesting the fire is likely to have started in that room. The order in which alarms were triggered can be used further to establish the path of propagation of the fire. However such information is not available for all premises.

The investigator may be required to excavate the scene and systematically remove debris in order to identify the possible origin. Once debris and other evidence can be collected the scene can be lightly cleaned to expose fire burn patterns. However, depending on the extent of fire damage, the seat of fire may have been destroyed, particularly if the fire has been burning for a significant length of time.

The growth of the fire, whether fast or slow, and its heat can be suggested by fire damage at the scene. Spalling of plaster suggests a rapid increase in temperature, though the quality of the plaster and fire-fighting efforts can distort the usefulness of this. Intense charring is indicative of a slow, smouldering fire acting as the source. Fire damage to glass can also suggest the heat of the fire. The rapid increase in temperature can cause clear breaks in the glass, whereas a very slow build-up of heat tends to cause the glass to soften rather than break. Examining the extent to which wooden structures have been charred can provide insight into the fire, as exposed wood chars at a rate related to the exposure time and amount of radiant heat.

There may be multiple seats of fire, which in some cases can indicate arson if the arsonist has started fires in numerous places. However burning wallpaper, curtains or debris can also produce apparently distinct ignition points. Due to the range of factors affecting the origin of a fire, it may not be possible to specify the exact point of ignition of a fire. Therefore investigators generally define a confidence perimeter or radius of error. This is an extended section somewhere within which is the seat of the fire, with the most probable origin placed in the centre of the circle. Generally, the radius of this circle will decrease as the investigator becomes more confident in establishing the origin.

Establishing the Cause
Determining the cause of the fire is often greatly aided by locating the seat of fire, at which point investigators can identify characteristics or artefacts associated with ignition. The investigator will aim to establish whether the cause of the fire was accidental, natural, deliberate or undetermined. Accidental fires generally involve no malicious human contact, with examples including the malfunction of an electrical appliance or an unattended candle. Natural fires include “acts of God”, such as lightning strikes. Deliberate fires are those ignited purposely by individuals, often with malicious intent, in an act known as arson. Finally, if the cause of the fire cannot be ascertained due to lack of evidence, it may be classed as undetermined.

Evidence directly linked to the fire may be found at the point of origin, such as fuel sources, incendiary devices, electrical appliances or pools of accelerant. In addition to examining the artefacts present at the scene, the lifestyle of individuals living or working in the building should be taken into consideration. For example, factors such as whether individuals were smokers, used candles or kept large amounts of possible fuel packages such as newspapers and magazines may be relevant.

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There are numerous indications of the deliberate ignition of a fire, also known as arson. Cases of arson are of particular importance to the forensic investigator, and such incidents may arise for a variety of reasons, such as insurance fraud, terrorism, in attempts to harm a person or their property, mental health problems, or to conceal a previous crime.

A particularly significant indication of arson is the lack of evidence suggesting an accidental or natural fire, though it is possible that the cause of even an innocent fire has been destroyed and cannot be ascertained. Signs of forced entry into the premises can suggest arson, displayed through broken windows, forced doors, tools found at the scene or disabled intruder alarms.

Flammable liquids are commonly used by arsonists to accelerate a fire, particularly patrol, diesel, kerosene and turpentine. The use of accelerants is suggested by extremely localised burning patterns with clear demarcation between burnt and unburnt areas, multiple seats of fire or trailing marks, and the detection of hydrocarbon vapours using sniffer dogs or hydrocarbon detectors. Flammable liquid containers may also be found at the scene. However it must be taken into account that flammable liquids may be present for innocent purposes, therefore it is necessary to determine whether such accelerants were stored on the premises prior to the fire. Other fuel packages may also be used, such as newspaper, which may be suspiciously piled up and ignited. If an incendiary device was used to ignite the fire, evidence of the device may be found amongst the debris. Furthermore if numerous devices were used, they could be found intact if they failed to detonate.

Investigators should attempt to ascertain the contents of the building prior to the fire. The removal of items from the premises, such as business stock or objects of sentimental or monetary value, is a strong indication of arson, commonly linked to cases of insurance fraud. The owner of the premises should be investigated and any possible financial or business problems searched for, which would provide further evidence in the form of a motive.

Fires are occasionally started to conceal a previously committed offence. However if the fire was ignited to conceal a murder, it is extremely unlikely that the victim’s body will actually be completely destroyed, as this would require temperatures of hundreds of degrees Celsius for 2-3 hours.

In some cases the arsonist may make attempts to shield the cause of the fire or attempt to make it appear to be natural or accidental. For example, they may start to fire close to an appliance or pile newspapers near a potential ignition source. Arsonists may block windows to shield the fire until it has developed, or conversely prop doors open to provide ventilation. They may also place objects to hinder entrance to the building and fire-fighting efforts.

In cases of suspected arson, it may prove beneficial to observe or photograph any onlookers. Arsonists have been known to return to the scene to watch the fire and the investigation. Certain indicators at a fire scene may not only suggest arson, but can also provide an insight into the possible motives of the individual responsible. People connected to the premises should be interviewed and investigated to search for any possible motives for arson.

Electrical Fires
When an electrical current passes through any material resistance will be encountered, producing some heat. Electrical wiring is usually produced and installed in such a way that any heat produced is relatively low and will be dissipated. However there are some occasions in which the heat produced can reach sufficient temperatures to cause ignition. Electricity is a common cause of accidental fires, often through the occurrence of an electrical arc.

An electrical arc occurs when two conductors come into contact following the insulation in the cable being damaged. This damage can occur for various reasons, particularly overheating, overloading, mechanical damage or manufacturing defects. If the cable becomes too hot, perhaps due to coiling of wires, heat will be unable to dissipate and the insulation may melt, allowing conductors to touch. Overloading occurs when more power is drawn through the cable than it is designed to handle, such as if too many plugs are inserted into one socket. This can also occur through the fitting of incorrect fuses or cable sizes. This will also cause insulation melting. Mechanical damage can occur through direct damage or continuous movement, weakening the cable at a certain point and thus allowing contact between the conductors. Similarly, damage may be the result of defects in the manufacturing process. Arcs are characterised by beading on the cable caused by the wire melting. It should be taken into account that although electrical arcs can lead to fires, fires can equally cause arcs.

If the suspected cause of the fire is an electrical appliance, the equipment must be thoroughly investigated, with a record being kept of details such as the brand, model and serial number. The expert must first conclude whether the appliance was turned on or off, whether it had a power supply, and whether the power supply was active or if the fuse had blown. Unfortunately an appliance which has caused a fire will most likely have suffered a great deal of damage and so confirming the cause of the fire may be extremely difficult or even impossible. It may be necessary to consult an expert for advice.

Upholstery Fires
The spread of a fire, the extent to which it grows and the pyrolysis products formed partly depends on the types of fuel available. In compartment fires in homes and other buildings, there will often be large amounts of upholstered furniture present, including beds, mattresses, sofas, armchairs and futons, all of which are a potential source of fuel. Upholstered furniture generally consists of a frame, filling material such as foam, and an outer covering fabric.

Various problems have been encountered with upholstered furniture in fires, particularly the flammability of materials used in their manufacture and the toxicity of materials used. In the 1970s-1980s a type of foam filling was used which produced toxic fumes when burned. The Furniture and Furnishings (Fire Safety) Regulations 1988 applied various fire resistance standards to upholstered furniture such as sofas, beds and armchairs. Following this legislation, modern upholstered furniture must include labels with fire resistance information. Furthermore, modern furniture is often produced using flame retardant textiles. For example, nitrogen and chlorine inhibit the burning rate of textiles and so are often used to treat fabrics. Other substances are added to increase the amount of charring and so create a heat barrier to prevent the fire from spreading further.

Flashover is a phenomenon known to occur in compartment fires following a series of events, eventually resulting in the compartment’s full involvement in the fire.

Radiation-induced flashover is one particular form of this. As a fire burns in the room and the fire plume cannot escape, a layer of hot gases produced by the fire rise and form at the ceiling, increasing the temperature of the upper portion of the room. Flameover may occur, which is the fast horizontal spread of flames. As temperature increases, the rate of heat radiation increases. Temperatures at this point can reach around 600oC, with radiant heat flowing down to floor level. Soon flames across the ceiling can reach between 750 and 850oC. At this point all available combustible materials in the room can reach their auto-ignition temperature and burst into flames. This process is known as radiation-induced flashover. Furthermore, if a compartment is breached through the opening of a window or door or due to structural collapse, the influx of oxygen can result in the occurrence of an explosion known as ventilation-induced flashover.

However flashover will not occur if there is insufficient fuel, inadequate heat production, too little ventilation or too great a flow of heat out of the compartment.

Outdoor Fires
When investigating an outdoor fire, there are various differences from compartment fires that must be taken into consideration. A fire burning on a flat, open surface will move outwards towards any available fuel whilst producing hot gases above the fire. Assuming the fire is surrounded by a similar fuel source and there is no wind to take into account, the fire will most likely spread in a circular pattern. A fire on a sloped surface will most likely spread in an uphill direction, provided there is a fuel source, producing a fan-shaped spread.

Evidence Collection & Analysis
In the collection of evidence during the investigation of a fire scene, the same rigorous preservation and anti-contamination methods used in crime scene investigation should be employed.

In cases of suspected arson, samples are collected from the incident scene for the analysis of accelerants. The use of accelerants is not always apparent, therefore investigators may need to use detection dogs or hydrocarbon sniffers to detect these volatile substances. Hydrocarbon sniffers are vapour detectors used to discover the presence of fuel and solvent vapours associated with flammable liquids. Early devices implemented treated paper or crystals which changed colour when exposed to hydrocarbons, whereas more modern devices are essentially portable gas chromatographs or flame ionisation detectors. However these devices can only ever act as a preliminary test for accelerants, as similar substances can also be produced through the thermal decomposition of various natural and synthetic materials that may be found at the scene.

Once likely regions have been located, fire test samples are collected from the suspected point of ignition. In addition to this, a control sample should also be obtained, which consists of the same material as that of the fire sample but collected from an area uncontaminated by the suspected fuel, and a negative control sample. When collecting samples of possible accelerants, surface samples may be collected however, in some instances, charring of floors may be too severe. In this case samples can be collected from grooves between or beneath floorboards or even from soil below the floorboards.

All samples containing potentially volatile substances should be stored in airtight containers such as metal containers, glass jars or impervious plastic bags. All samples should be stored and submitted separately. The analysis of volatile samples is generally conducted using a technique known as headspace analysis. A common method used in the employment of headspace analysis uses a piece of activated charcoal or a similar adsorbent material which is stored in an airtight container with the volatile sample. Volatile compounds are drawn into this material either passively or dynamically and later desorbed for analysis.

Gas chromatography is the technique most commonly utilised in the analysis of fire debris. This allows for volatile substances, whether from bulk or trace samples, to be separated, displayed in the form of a chromatogram, and identified. The technique is also able to isolate and identify mixtures of various compounds. The use of gas chromatography not only permits samples to be identified, but can also allow for numerous samples to be compared to establish whether or not they are the same substance.


Daeid, N N, 2005. Fire Investigation. Florida: CRC Press.

Fairgrieve, S I, 2008. Forensic Cremation: Recovery & Analysis. Florida: Taylor & Francis Group.

TC Forensics. [online] Available at: [