The SouthInc fire spread quickly along steep, rugged terrain in the Fillmore area amid Sunday's hot temperatures.
But firefighters believed they were getting the upper hand and had stopped the forward progress of the blaze by Sunday evening after it had charred 131 acres.
"Cause of the fire has been determined to be target practice on private property," the Ventura County Fire Department said.
Air tankers that were dropping water on the flames have been sent home.
Three firefighters suffered minor injuries battling the flames.
Ground crews are expected to remain on scene for some time putting out hot spots.