Forensic Arson Investigation Techniques


Arsonists cause property damage, injury and death. According to the National Fire Protection Association, approximately 282,600 fires were set intentionally in the U.S. each year from 2007 to 2011. Arson investigation involves a comprehensive knowledge of both fire and human behavior. Arson investigators must find the cause of the fire, analyze the evidence, and understand the victims to help identify the arsonist.

Fire Pattern Analysis

  • Determining the origin and cause of the fire are critical to determining if a fire was set intentionally or if it was accidental. If it was set deliberately, knowing how it was started helps investigators while investigating suspects. One technique that can be used to determine fire origin is to examine fire patterns, states National Geographic. The investigator examines burn patterns on the walls, floor and furniture in the structure. However, this method is open to interpretation and bias, advises the California Attorneys for Criminal Justice Forum. When flashover conditions occur and objects combust due to high heat, normal burn patterns can be distorted and lead investigators to an incorrect conclusion. The scene, including burn patterns, must be photographed for a record of evidence.

Witness and Victim Interviews

  • Investigators usually interview the person who called in the fire and any other witnesses and bystanders. These individuals may have information about where or how the fire started and if they saw anyone leaving the scene. In addition, investigators interview the owner and occupants of the building. First responders might also provide information about how the scene looked when they entered the building. Investigators must not lead the witnesses by sharing their suspicions of the fire's cause or origin and should keep witnesses separated if possible so that they are not influenced by other statements. When possible, the interview should be recorded, or the investigator may ask the witness for a written statement.

Physical and Trace Evidence

  • Investigators on the scene collect evidence and make every attempt to preserve its integrity. This includes fingerprints and footprints, blood, and evidence of a fuel or any device that might have started the fire. Wet weather conditions might damage or degrade trace evidence, fingerprints, footprints and tire tracks. Investigators should document when and where evidence was collected and document transferring the evidence to the lab. Failure to do so interrupts the chain of evidence and may cause it to be inadmissible in court.

Laboratory Analysis of Evidence

  • Laboratory technicians analyze the evidence collected during an investigation. They help identify burned and unrecognizable objects and analyze trace evidence. Chemical analysis techniques help determine if an an accelerant or chemical was used by the arsonist and if so, identify it. Some things arsonists use to start a fire include gasoline, potassium perchlorate and sulfur. If the fire resulted in a death, the medical examiner must perform an autopsy.

Background and Financial Investigation

  • Conducting background investigations helps the investigator determine the possible motive for setting the fire. Background checks might be done on the victim, the property owner and the person who called in the fire. Investigators also check their financial information, including debt, tax records and insurance policies and claims.

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