It was a clear day, no lightning, no powerlines, roads etc in the area they started. It was arson. And guaranteed someone has already been to the ignition point and has seen that it was human caused.
That’s my understanding, as well.
One firefighter said he could smell the kerosene (or maybe diesel, he wasn’t sure) at the South Park fire.
We had a small fire near us that started on a clear day, but there had been lightning in previous days, sometimes they can smolder.