Investigation of Fire Scenes
Fire scenes are among the most difficult places to investigate. In many cases, a building may be completely destroyed by fire and upper floors may have collapsed onto the lower parts of the building. The fire department may have attended the fire and attempted to suppress it. This usually involves using many thousands of gallons of water to cool and douse the fire. The scene is usually dark, as the electricity would have been cut off. Fire scenes can be exceedingly dangerous. There may be smoldering embers buried in the debris. The structure of the building may be weakened so that walking in the building can be hazardous. There is the danger of parts of the building collapsing on the investigators. At the same time, it is necessary to avoid disturbing the scene as much as possible so as to not dislodge or contaminate potential evidence. Figure 1 shows the damage that can be caused by an indoor fire. Fire scenes can be properly investigated only by highly trained fire scene investigators. They must be aware of the causes of fires, burning patterns, how different materials react to fire, the characteristics of points of fire origin, how fires normally proceed through a structure, unusual fire characteristics, and the effects of fire sup pression on the scene. In addition, many experienced investigators are knowledge able about appliances, especially furnaces and hot water heaters, and how they fail or can be tampered with. The main duty of the fire scene investigator is to determine the cause of the fire. The most important piece of evidence in this determination is the point of origin of the fire. Finding the point of origin is crucial to determination of the cause because it will permit the investigator to determine if a fire was accidental or incendiary. Proper investigation of a fire scene involves many important processes. The investigator must proceed in an orderly, methodical way, and must make accurate, thorough records of the investigation through still or video photography and good note taking. Like many criminal examinations, the investigation of a fire scene starts with a general examination of the scene and gradually focuses on the room or origin and then the point of origin. If the fire scene is a building, then the investigation would normally start with the exterior of the building and work toward the point of origin inside the building. If the fire is outside, such as forest, then the investigation proceeds from outside the damaged area into toward the area of origin.
FIGURE 1 Indoor fire scene. The fire started in the couch and burnt up and out.
Points of entry and exit Conventional wisdom is that the most important piece of evidence about a fire is the point of origin or ignition of the fire. As will be shown later, this is crucial evidence, but in the case of a deliberate fire, the perpetrator may have started the fire from within the building, rather than remotely. In this case, there would be a point where this person entered the building—the point of entry. In order to leave the arsonist a route of escape, there may also be a remote point of exit. Of course, the points of entry and exit may be the same. These locations may be very important in determining who started the fire because there may be trace evidence left by the perpetrator upon entering or exiting the fire scene. Even if the fire is severe, these locations are often remote from the point of origin and the most severe damage from the fire. This is because the perpetrator would not want to be detected when starting the fire. In such cases, the trace evidence may be well preserved. Physical evidence that is found at points of origin and exit of fire scenes includes finger prints, shoeprints, hairs and fibers, soil, and even blood. These items can provide circumstantial or direct evidence of the identity of the arsonist and they must not be overlooked.
Point of origin Certainly, the points of entry and exit can provide important clues about who may have started an incendiary fire. The most important evidence about how a fire was started is the point of origin or ignition. This is the location where the initial ignition took place. If there are accelerants used in a fire, they are most likely to be here. As a general rule, the most intense burning and damage are found in the area around the point of origin. Deviations from this can occur for a number of reasons, including wind direction, efforts at fire suppression, locations of fuels and/or accelerants, drafts, etc. The point of origin of a fire can be seen in Figure 18.5.
Locating the point of origin There are generally a number of characteristics present at the point of origin of a fire. They include the following:
• Low burning—Fires generally start in a low area of a building. Arson fires are seldom started at a high place because the perpetrator may not have a safe point of exit and the damage will generally not be as great since fires burn in an upward direction.
•V patterns—If the point of origin is near a wall or corner of a room, smoke damage on the wall(s) usually occurs in a “V” shape. This is not universally true and there may be other areas in the building where V patterns occur but this burn pattern can be good evidence of the point of origin. Figure 2 shows a classic “V” pattern in an interior fire.
• Wood charring—The depth of wood charring depends upon the intensity of the heat near the wood and the time of exposure. Often, wood near the point of origin of the fire will have charring to a greater depth than elsewhere in the building, although this is not always true, because there may be other points where the fire burns hotter or longer than at the point of origin.
• Spalling of plaster or concrete—Spalling is the destruction of a surface due to heat or other factors. In the case of concrete, the spalling may be explosive owing to trapped moisture and expansion of the concrete. Spalling usually occurs most where the heat is most intense. One of the pervasive myths about spalling is that it is only caused by the presence of an accelerant. This is not true. There can be enough localized heat from a fire in the absence of an accelerant to cause spalling.
• Material distortion—Metal and glass may melt or distort owing to high heat. Since melting points of many of these materials are well known, such destruction may indicate the approximate minimum temperature of the fire at that point. If the fuels that are supporting the fire are not capable of reaching that temperature during burning, an accelerant may be suspected.
•Soot and smoke staining—The amount of soot present in a fire may indicate the point of origin and the direction of travel of the fire. If there are indications that soot was first deposited on a surface and then burnt further, this may be good evidence of the point of origin.
Indications of an arson fire In order to definitively determine the cause of a fire, the investigator must find the fuel that was first ignited to start the fire, the source of the heat that got the fire started, and how the two came together. Finding the point of origin of a fire is usually a necessary condition of determining if a fire was deliberately set for malicious pur poses, but it may not be sufficient. There are a number of factors that may be present that would indicate that a fire was arson.
• The presence of an accelerant—If an accelerant is present at or near the point of origin of the fire, it usually, but not always means that the fire is arson. There may be some cases where a can of gasoline is stored in a building and the fire starts by some other means near the can and it subsequently becomes involved in the fire. The investigator would find the accelerant but, in this case, a finding of arson on that basis alone, would not be correct.
• Elimination of natural or accidental causes of a fire—This is a necessary condition of determining that a fire is arson. If the point of origin is found and there is no evidence that the fire was started by natural or accidental causes, then the fire must have been incendiary. This may be difficult to determine at times. For example, a furnace may malfunction, leading to a fire. The furnace, being at the point of origin, will be damaged or destroyed. It must be determined whether the furnace malfunctioned accidentally or it was due to deliberate tampering. This determination can be very difficult if there is extensive damage to the furnace.
• Fire trails—In order to enable a fire to travel rapidly in particular directions, a fire trail may be employed. This can be accomplished by pouring an accelerant along a floor in the desired direction. The result will be an uneven, intense burn along the fire trail. This would not be seen in an accidental or natural fire, thus fire trails of this nature are almost always arson. The investigator must be careful to not mistake a fire trail pattern caused by a burnt, plastic carpet runner or wear in a carpet or floor. Usually, such “fire trails” are much more regular in shape than those caused by pouring an accelerant. See Figure 3 for a fire trail.
• Multiple points of origin—Pouring some accelerant in each room of a build ing and then starting multiple fires, one in each room, is a tactic often used by arsonists to involve a building in a fire quickly. The investigation of this fire would show multiple points of origin and it would be easy to classify this fire as incendiary. This is usually, but not always the case with fires that have more than one apparent point of origin. There are exceptions. For example, suppose the electrical wiring inside a building was faulty and perhaps an overload occurred at an outlet. This might lead to overheating of the wiring and insulation at several points inside the walls of the house. Fires might start at some or all of these points of overheating. The investigation of the fire would show multiple points of origin, but the fire would not be arson (unless the original overload was made to happen deliberately). A good fire scene investigator can usually tell whether electrical wiring burned because it got overheated or it came into contact with fire that started elsewhere. Aside from such exceptions, multiple points of origin are generally indicative of incendiary fires.
FIGURE 2 Point of origin of a fire showing the classic “V” burning pattern as fires burn up and out.
FIGURE 3 A fire trail made by pouring a flammable liquid on a floor. Note the irregular pattern of burning caused by pouring the liquid out of some container such as a bucket.
