The culprits apparently use non-traceable disposable cell-phones, which once purchased ... they hack the wiring so as to ignite large pieces of dried cloth that were previously soaked in gasoline.
Once the "device" is prepared, the "illegals" are sent to designated areas of interest where they plant the phones to be activated at a later time. Some cell phones last for days without being charged, so it's difficult to catch them in the act.
I think vigilante justice is sorely needed to deal with these criminals.
These fires are just another form of "Disaster Capitalism".