Origin and Cause Determination
Origin determination
Origin is the location where the fire began; cause is the circumstances that brought the two together. Work from the least damaged to the most damaged, reconstruct the pre-fire scene, and analyze fire patterns: V-patterns, hourglass patterns, char depth, line of demarcation, calcination of gypsum, concrete spalling, melted metals, and heat shadowing. No single indicator is conclusive - alligatoring and "deep char means a hot fire" are unreliable, and spalling can result from rapid cooling. Identify a competent ignition source with the energy, temperature, and duration to ignite the first fuel, and show that it contacted that fuel.
Cause classification
NFPA 921 classifications are accidental, natural, incendiary, and undetermined. Undetermined is a legitimate, defensible finding when data cannot support a cause. The negative corpus - concluding incendiary because no accidental cause was found - is expressly discouraged. Accelerant detection requires laboratory confirmation; a pour pattern may instead be melted synthetic material, a failed container, or a ventilation effect.
TRAP: Questions reward the confident answer. The correct choice is the one consistent with physical evidence and the scientific method, not the most dramatic.
RULE OF THUMB: Your origin hypothesis must explain the greatest amount of damage, match the pattern geometry, and be consistent with the first material ignited and with compartment ventilation.