Arson Investigation

Arson investigation is a collective term for the examination of any fire-related incidents, conducted by forensic investigators once fire-fighters have extinguished the fire and the area has been declared safe to enter. The Federal Bureau of Investigation defines arson as “Any wilful or malicious burning to attempt to burn, with or without intent to defraud, a swelling house, public building, motor vehicle or aircraft, personal property of another, etc”. The initial stage of the investigation involves ascertaining the origin and cause of the fire or explosion, simultaneously establishing whether the fire was accidental or purposeful. Arson is typically graded by the seriousness of the fire. First degree arson involves homes, schools and churches. Second degree involves unoccupied structure and vehicles, whereas third degree involves the burning of personal property.

Arson investigators must understand the science and behaviour of fire in order to effectively investigate the scene. All forms of fire and explosion are essentially a type of combustion, a chemical reaction based on oxidation. Combustion occurs through the combination of a fuel, an oxygen source, and heat. A fire will only continue when a continuous oxygen source is present, and most will extinguish themselves with less than 16% oxygen. A knowledge of the colour and state of certain materials at high temperatures will often prove useful to the investigator, as certain substances will burn with a specific colour smoke. For example, gasoline with produce a yellow or white flame with black smoke, whereas wood produces a yellow or red flame with grey or brown smoke.

When examining the scene, the investigator will look for certain damages caused by the fire. These may include sooting, which is smoke damage, scorching, and charring to surfaces. The search may often begin at lower levels where, as fire travels upwards, the origin probably was. Lingering heat, flaking and charring, distorted plastic metal and glass, and ceiling and structural damage are all common results of fire. The path in which a fire will take when spreading is influenced by numerous factors, including fuel, air currents, walls, stairways, falling burning debris, and the efforts of fire-fighters. Doors and windows may also be an indication of arson. The arsonist may have propped them open in order to ventilate the building and spread flames. Alternatively, if the doors and windows are blocked out, the perpetrator may have been attempting to conceal the fire until it had fully developed. The presence of some form of fuel is a strong indicator of arson, as arsonists will often use an accelerant to speed up the burning process when starting a fire. Irregular pooling marks on the floor are indicative of a fuel being placed intentionally at the scene, particularly if there are multiple sites of origin. The investigator may use hydrocarbon detectors to locate the presence of fuels used at the scene. Arson dogs will often be brought in to detect traces of accelerants, not only locating them but also situating the area in which the fuel is strongest.

Substances used as fuels in the starting of fires are classed by ASTM classification as followed:

Class 0: Miscellaneous. Oxidated solvents, isoparaffinic, normal alkane, and aromatic products.

Class 1: Light petroleum distillates. Includes some lighter fluids and camping fuels.

Class 2: Gasoline.

Class 3: Mid-range petroleum distillates. Includes some paint thinners and charcoal lighter fluids.

Class 4: Kerosene.

Class 5: Heavy petroleum distillates. Includes some jet fuels, diesel, and fuel oils.

Any substances found at the scene can be analysed using gas chromatography and mass spectrometry. When such samples are detected and recovered from the scene, they should be transported and analysed in a reasonably short period of time to avoid the loss of volatile evidence, especially as many fuels will readily evaporate.

In cases of arson for monetary gain, such as the destruction of a business to claim insurance money, a number of clues may prove arson. With businesses, the arsonist will often move all stock to another location so that it is not lost in the fire, indicated by the lack of stock debris present at the fire scene. When dealing with insurance claims on a burned property, investigators should conduct a background check on the owner of the property in order to detect any signs of debt that might suggest motive. Such arsonist will also remove items of sentimental value from the scene before starting the fire. The investigation of a wildfire is often more complex, due to the larger area and the additional environmental factors involved. Wildfires follow a specific pattern of behaviour, spreading outward in a V or U-shaped pattern, moving with the direction of the wind, and moving faster uphill.

Some of the most common motives for arson include profit, revenge, the concealment of another crime, pyromania, and attention seeking. Arsonists have been known to remain at the scene and join crowds of onlookers to watch the fire, therefore it is often beneficial to note or photograph individuals present in the crowd.