Bob:
You are exactly right. You do not need to know the precise details of the ignition scenario to know that a fire should be classified as accidental.
I believe the appropriate way to approach a fire (in contrast to what is written in 921) is to PRESUME the fire is accidental. Give the fire the PRESUMPTION of innocence, and you will afford your fellow citizen the same presumption.
If the fire was not an accident, there will be evidence of that. The concept that all fires are presumed to be accidents until proven otherwise is actually the LAW in 18 states.
I for one am growing weary of investigators who call "undetermined" fires incendiary, simply because they are unable to identify an accidental cause.
John Lentini, CFI, D-ABC
Fire Investigation Consultant
Florida Keys
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www.firescientist.com]