Fire investigations, especially where arson is concerned, is grounded on the Revised Penal Code. Specifically, the laws related to arson can be found in Book Two, Chapter Eight (Articles 320-326).
Point of origin of the fire
Motive of the arsonist
Prime suspects (and the prima facie evidences)
Tell-tale signs of arson
One of the most important things to establish in an arson investigation is the point of origin of the fire. If an obvious point of origin could not be found nor confirmed during inspection of the fire scene, the investigator must turn to witnesses for information.
The witnesses must be questioned on:
Their identity
What attracted their attention
Time of observation
Their position relevant to the initial fire
Exact location of the blaze
Size and intensity
Speed of the spread
Color of flame and odor of the fire
Any other persons found on the scene
An investigator would also do well to note the following fire-setting mechanisms:
matches
candles
electrical systems
mechanical means
chemical methods
There are two types of fire-setters: those who have motives and those who don't. The latter could be mentally ill persons who need medical attention.
For those with motives, they could have set the fire for economic gain, to conceal a crime, as a punitive measure, or to intimidate.
The development of prime suspects could be derived from the information gathered during inspection and from the testimony of witnesses. The following techniques may serve the investigation:
Search of the scene for physical evidence
Background study of the people involved with the building
Interviews or interrogations of the discoverers of the fire
Surveillance
The following, if observed, may be signs that an arson has occurred:
Burned down building
Separate fires
Suspicious color of smoke
Odd color of flames
Extreme temperatures during the blaze
Questionable smoke marks
Unusual fire size
Unnatural direction of fire spread
Severe intensity
Dubious odors
Questionable items left behind in the burning building