Fire Investigation and Cause Determination
Origin, cause, and the scientific method
Fire investigation is a systematic process: secure the scene, document, determine area of origin, then point of origin, then cause, then responsibility. Cause classifications are accidental, natural, incendiary, and undetermined. 'Undetermined' is a legitimate finding, not a failure.
Know NFPA 921's scientific method: identify the problem, collect data, analyze the data, develop a hypothesis, test the hypothesis, and select a final hypothesis. Fire patterns such as V-patterns, char, heat lines, spalling, and melted materials are indicators, not proof. Traps include treating a pour pattern as certain proof of an ignitable liquid (it may be a melted synthetic fuel load or a ventilation artifact), treating alligatoring as reliable, or declaring incendiary before ruling out accidental causes.
Evidence handling matters: chain of custody, packaging, photography, and scene security. Investigators coordinate with law enforcement, but fire cause determination and criminal investigation are distinct.
Worked example: A room shows a V-pattern on one wall, heaviest char low, and a burned appliance cord. The V-pattern suggests the area of origin, but the cause requires examining the cord, the appliance, and electrical service. Do not conclude electrical from the pattern alone.
Rule of thumb: Origin before cause; a hypothesis must survive testing against all damage, not just the most dramatic pattern.