https://www.mercurynews.com/2019/06/07/cal-fire-investigation-determines-cause-of-ranch-fire/

Last year’s Ranch Fire, still considered to be the largest wildfire in state history, was started by someone hitting a metal stake with a hammer, the California Department of Forestry and Fire Protection (Cal Fire) reported Thursday.

According to Cal Fire, the fire was first reported by a 911 caller around 12 p.m. July 27, 2018, who said there was a vegetation fire near Potter Valley Road and Highway 20 in Mendocino County. The fire was later named the Ranch Fire and grew rapidly, eventually consuming 410,203 acres and causing the death of one firefighter and injuries to at least three others. Cal Fire notes that no criminal charges were filed as a result of this investigation.