A spark from a stray bullet hitting a rock led to the fire that burned a swath across Mount Diablo, according to an investigation by the Contra Costa County District Attorney's Office.
No charges will be filed in the case, however.
The Sept. 8 fire burned more than 3,100 acres and took a week to contain. The DA's investigation said a shooter on private property on Morgan Territory Road in an unincorporated area of Clayton sparked the fire.
Part of the property was designed and used as a rifle range, according to DA Mark A. Peterson.
"While target-practicing, one of the steel projectiles fired by the shooter impacted against a rock, caused a spark, and started a fire in an area of dry grass," D. A. Mark A. Peterson said in a statement. "Immediately upon seeing the flames, the individual attempted to extinguish the fire but was unable to do so. He contacted a family member who called 9-1-1."
Peterson said the individual, who was not named, was cooperative with firefighters and helped them as they tried to stop the fire from spreading.
"Within 30 minutes of the 9-1-1 call, fire personnel had arrived and had the fire contained to half of an acre," Peterson's statement said. "Unfortunately, as the fire was being 'mopped up,' a 'spot fire' from the original fire started about 1,000 feet away from the original fire, causing a second fire which grew out of control."
That spot fire ultimately forced the evacuation of more than 75 homes, injured at least two firefighters and prompted health advisories in Contra Costa, Alameda and Santa Clara counties.
But, Peterson said, the shooter will not be charged.
"There is no evidence the fire was intentionally started. There is no evidence that the use of the involved rifle or ammunition in that area was in violation of state law," he said in his statement. "There is little evidence that someone would know or should know that a discharged projectile would or could cause a fire in this way."
While Peterson noted his office is "fully aware of the tremendous harm to property and the environment caused by the fire," he noted in his statement that the key question is whether the man could know that it was "reasonably foreseeable" that his shooting would cause a fire.
"It is the determination of this office that no criminal liability can be attached to the cause and origin of this fire. Therefore, under the law, this office will not be filing any criminal charges regarding the fire," the DA said.