A fire broke out at the defunct Harwood lumber mill in Branscomb Wednesday afternoon and continued to burn into the evening.
The fire at the mill site included a building that was fully involved, according to the California Department of Forestry.
The fire was reported at 2:21 p.m.
Multiple fire agencies and 20 trucks responded to the first and were still battling the blaze three hours after it began.
The 58-year-old mill has been largely dismantled since the family owned company declared bankruptcy in 2007.
Equipment and the buildings themselves were auctioned off in November.
The cause of the fire remained unknown Thursday night.