MENDOCINO COUNTY — Cal Fire announced Thursday the cause of the Ranch Fire was by a spark or hot metal fragment landing in a receptive fuel bed.
Investigators said the spark or hot metal fragment came from a hammer driving a metal stake into the ground.
No charges have been filed.
The Ranch Fire, which started in the early afternoon of July 27, 2018, burned 410,203 acres in Colusa, Glenn, Lake and Mendocino counties.
By acres burned, the Ranch Fire is the largest wildfire in California History.
The fire destroyed 280 structures.
One firefighter was killed and three others were injured.
The Ranch Fire was one of two fires that made up the Mendocino Complex fires.
The Ranch and River fires burned a total of 459,123 acres.