Fire investigation is a field of forensic sciences dedicated to the determination of the origin and the cause of a fire. Determining the origin of the fire answers the question, "Where did the fire start?" Determining the cause of the fire answers the question, "Why did the fire start?"
In order for a fire to occur, three conditions must be met. There must be a combustible (fuel), an oxidizer (oxygen), and a sufficient heat energy source (source of ignition). The key elements that determine the cause of the fire rely on the determination of the first material ignited and of the source of ignition. Heat energy can be produced by mechanical, electrical, chemical, or radioactive means. The role of fire investigation is to identify the event that brought together the combustible, the oxidizer, and the source of ignition that started the fire.
Causes of fire can be natural, accidental, deliberate, or undetermined. Natural causes of fire include all the actions of nature that can cause a fire, such as lightning from the sky or lava from a volcano. Accidental fires include both fires that are caused by a negligent human intervention or by accidental occurrence without the necessary presence of a human being. Deliberate fires are caused by the intentional intervention of a human being. However, not all deliberate fires are arsons. For example, if one sets fire to backyard debris, it is deliberate but may not constitute arson . Arson is a legal term that can greatly vary from one country to another or from one state to another. In many instances, fire investigators will not be able to determine where and/or why the fire started, and the cause is classified as undetermined.
The determination of the origin of a fire is made based on observations of smoke, heat, and burn patterns at the fire scene. Fire evolves following the laws of physics and chemistry. While combustion is a complicated phenomenon that can be random in some instances, it is most often possible to determine the direction of fires. By tracing the direction backward, it is possible to find the origin of the fire. Once the origin of a fire is found, it is necessary to determine its cause.
One of the most important objectives in fire investigation is determining if a crime occurred in connection with the fire. If the fire is due to arson, it is extremely important to quickly determine that a crime occurred and gather evidence , so that an arrest may be made. If an accidental or natural fire occurred, it is important to determine how it started, so that the proper measures can be taken to prevent similar fires and to protect lives and property from future damage. In addition, in the case of accidental fire, it is important to determine the exact cause of the fire, because people or companies might bear a responsibility in the loss. The responsibility might be civil or criminal and carry an important financial burden. Many criminal laws also allow charges to be brought against people who accidentally created a fire.
Fires are investigated by fire investigators or criminalists. Fire investigators come from many different backgrounds. In some European countries, fire investigators are commonly part of the police department, usually the crime scene unit. In English-speaking countries and in the United States in particular, it is often the fire department that employs fire investigators. Also, almost every state in the United States has a state fire marshal's office, which specializes in fire investigation. Furthermore, the Bureau of Alcohol, Tobacco, Firearms, and Explosives has a strong fire investigation practice and can dispatch the National Response Team within 24 hours almost anywhere in the United States to support state or local investigators.
In many instances, fire investigators need to rely on the knowledge of special experts such as electrical or mechanical engineers, forensic scientists, or chemists. For example, if the fire investigator suspects that liquid accelerants were used, he/she can collect fire debris to be sent to the forensic laboratory in order to search for such liquids. If an electrical apparatus is suspected to have caused a fire, an electrical engineer might help in the evaluation of the circuit or apparatus. If the failure of a shaft in a factory is suspected to have created the blaze by overheating, a failure analyst or mechanical engineer might be needed to determine the exact cause of the failure of the shaft.
Fire investigation is a harsh job, as it consists of working in dangerous conditions around burned structures or vehicles and in atmospheres containing many contaminants that are detrimental to the lungs and health. Fire investigators must often dig through debris from fire scene in order to see burn patterns. Often, it is necessary to reconstruct the fire scene in order to determine the pre-fire conditions.
see also Accelerant; Exothermic reactions; Explosives; Gas chromatograph–mass spectrometer.