After a fire is put out and CSI technicians or arson investigators arrive to inspect an arson crime scene, the first thing they want to know is where the fire started. They will look for clues to determine its origin.
Finding the point of origin of a fire requires knowledge of how a fire moves through a structure. In general, fires spread sideways and upward from the point of origin, but that pattern can change due to structural and decorative elements of the building. For example, stairs can cause a fire from one direction, and chemicals in man-made carpets can cause abnormal burning patterns. Typically, the greatest amount of damage occurs near the point of origin. The CSIs look for evidence of lighters or accelerants as possible clues that suggest a point of origin.
Other factors that can influence or hinder CSI’s efforts to determine a point of origin are open windows, stairs, or the materials used to construct or decorate the building.
After determining the point of origin, a CSI can sometimes retrace the path of the fire even when the building has sustained severe structural damage. On the other hand, backing up along the path of the fire can reveal the point of origin.
Looking for a V-pattern in the burnt material is another way to find the point of origin. It is the tendency of fire to rise and spread so that it burns a wall or other vertical surface in a V-shape, with the tip of the V located at the origin of the fire.
Fuel containers and other flammable liquids can not only make it difficult to find the true point of origin, but also to locate arson-related accelerants, simply because they are also accelerants. In addition, an arsonist may have started multiple fires within a building or spilled a path of liquid coal or other accelerated liquid through or around the structure, thus causing a fire with more than one point of origin.
By evaluating the effect of fire on structural materials, CSIs can estimate the intensity of the fire at any particular location. Under extremely intense heat, the steel beams bend and the glass melts at about 1500 degrees Fahrenheit. Cracking and peeling on floors and walls are indicative of very hot places. Similarly, wooden beams, walls and floors can burn and leave a pattern similar to the skin of an alligator. When this occurs, the smaller scales tend to be closer to the hottest point of the fire.
If smoke detectors are present throughout the structure, the timing of each alarm can help CSIs determine the path the fire traveled through the entire building and locate the point of origin.
Liquid and volatile fuels present special problems for fire investigators because they spread more quickly and take the shape of their containers. If an arson perpetrator spills paint thinner on the floor, the paint thinner spreads across the room, down the stairs, and oozes into the baseboards. By igniting the paint thinner, the fire follows the liquid and spreads immediately, causing the point of origin to spread widely.
Volatile fuels such as methane gas diffuse in all directions until their containers are filled. By igniting this fuel source, these containers can explode. Finding an exact point of origin in this scenario is impossible.