The primary responsibilities of the fire investigator are to determine the origin and cause of a fire scene. Fire investigators use the standard guide as set forth in NFPA 921 and determine a fire cause in one of four categories:

1. Incendiary – Fires that are intentionally and maliciously set
2. Accidental – Fires caused by non-malicious electrical, mechanical, chemical, nuclear and/or careless means
3. Undetermined – Fires that may have more than one possible ignition source and therefore cannot prove one definitive source of a single ignition point
4. Natural – Fires ignited by acts of nature, such as lightning