As a long-time employee of the largest and most respected C&O Engineering firms the country, I’ll just pass on the following facts gleaned from 12 years experience in this field.
A) In some fires it is easy to determine the cause and origin (C&O) simply because the fire is extinguished before much damage is caused. One the fire has grown to a significant size, this is no longer true. “Significant size” is hard to define, but once it gets to the size that a room or attic is burnt over, it is probably too late to be “easy”. I will refer to these fires as “significant” fires for lack of a better term.
B) In most significant fires, short of expending money like it was water, it is not possible to determine the C&O of a fire. When my firm investigates a fire, we commonly spend over $50,000 ( some times over $250,000) and hundreds of man hours (by Ph.D. Mechanical, Chemical and Combustion engineers), including burn test on suspected components (and sometimes entire buildings), and still frequently find the cause of the fire is undetermined or can be narrowed only to a few likely causes. Many C&O investigators claim they do the same with a couple of hours of investigation at the scene. Not likely.
C) When a fire department reports the fire was electrical or smoking related, this is normally (but not always) code for “we don’t have a clue what caused the fire.”
D) When a building burns to the ground, and someone tells you they know the cause, they are most lying, fooling themselves or started the fire themselves.
E) If n C&O investigators review a significant fire scene, you have about a 90% chance of getting n different causes.
F) Fire department C&O investigators are under a lot of pressure to provide a cause, so I should give them a break. They try hard, but they do not normally have the time, money, training or education to resolve the cause of a significant fire (Obviously there are exceptions).