"How often have you heard the phrase 'back to the basics'? It seems as though every time you turn around you are being instructed to go 'back to the basics,' whether it's with our children and their math homework or it's in the fire service with establishing a water supply, advancing a hose line, or conducting ventilation. The 'basics' are those tasks that you need to complete first, and they must be completed every time. Fire investigation is no different. You have heard about the fire tetrahedron that consists of heat, fuel, oxygen and an uninhibited chain reaction. In fire investigation, in order to determine an area of origin and a cause, you need to identify the source of ignition, the first material ignited, and the ignition sequence or the event that brought them together. Seems pretty basic! It wasn't all that long ago that an investigator's consideration of the fire tetrahedron stopped at this point: A fire occurred, so all the components of the fire tetrahedron must have been present. This is where the 'back to the basics' comes into play."
Coffee Break Training - Fire Investigation Series No. FI-2014-2
United States Fire Administration: http://www.usfa.fema.gov/