NFPA 921-based fire investigation methodology
This domain tests the NFPA 921-style process: a systematic, hypothesis-driven investigation rather than a story built from the most dramatic damage. Expect questions on origin versus cause, data collection, hypothesis testing, cause classification, and the limits of fire-pattern evidence.
Core methodology
- Define the problem, collect data, analyze the data, develop hypotheses, test each hypothesis against the evidence, and select the best-supported one.
- Keep origin and cause separate. Origin is the location where fuel, oxygen, and an ignition source interacted; cause is the circumstances or agency that brought them together.
- Use all available data: fire patterns, witness statements, first-responder observations, service records, electrical findings, laboratory results, and scene geometry.
- Fire patterns are indicators, not conclusions. Ventilation, flashover, suppression, and collapse can create misleading patterns.
- A valid cause classification uses the edition’s defined categories; “undetermined” is an acceptable finding when evidence is insufficient, not a failure.
- Avoid a negative corpus: never declare a cause simply by eliminating alternatives without affirmative evidence for the remaining one.
Worked example
An investigator finds the heaviest burning near a front-seat area and a melted aftermarket phone charger. The investigator first hypothesizes an electrical cause. Testing requires comparing the charger’s failure mode with circuit protection, battery condition, and fire direction. If the charger’s damage is consistent with external fire exposure, and no energized-source evidence remains, the electrical hypothesis weakens. If the available data cannot distinguish a discarded smoking material from an electrical fault, the correct conclusion may be undetermined rather than a confident cause.
Traps that cost marks
- Stopping at the first plausible hypothesis instead of testing alternatives.
- Declaring “electrical” from melted conductors or “incendiary” from irregular patterns.
- Confusing origin with cause in the final report.
- Treating undetermined as an admission of poor work.