There have been plenty of lightning-caused fires (including the recent fatal Arizona fire) and it's fairly easy to confirm now with nation-wide real time lightning detection networks.
A bit of a dirty little secret is that many of the arson forest fires are set by....firefighters.
Main thing behind the claims of jihadists starting fires is that people really, really, really want jihadists to have started fires.
>> “A bit of a dirty little secret is that many of the arson forest fires are set by....firefighters.” <<
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I can think of four that were, and there are also several that were set by USFS rangers.