A series of wildfires that destroyed thousands of structures and killed 18 people in Northern California last year were caused by downed power lines, state officials said Friday.

Investigators with the California Department of Forestry and Fire Protection, or CalFire, determined that power lines owned by Pacific Gas & Electric were at fault in 12 of the more than 170 wildfires that ravaged the region in October, torching more than 240,000 acres.

The report, which included fires in Mendocino, Lake, Butte, Sonoma, Napa, and Humboldt counties, found that in many cases the fires started after trees or branches fell onto PG&E power lines. In the Sulphur Fire, which burned 2,207 acres and destroyed 162 structures in Lake County, the failure of a PG&E power pole caused power lines and equipment to fall to the ground, investigators said.

CalFire has referred investigations to local prosecutors in eight of the 12 fires due to what it said was evidence of possible violations of state law.

PG&E said in a statement that it believed "our overall programs met our state's high standards."

"The safety of our customers, their families and the communities we serve is our most important job," the statement said. "The loss of life, homes and businesses in these extraordinary wildfires is simply heartbreaking, and we remain focused on helping communities recover and rebuild."

Investigators are still working to determine the cause of the remaining October fires, including the Tubbs Fire, which decimated entire neighborhoods in Santa Rosa and killed 22 people, as well as fires that burned in Southern California in December.

Mike Mohler, deputy director of communications for CalFire, told BuzzFeed News it was taking longer to investigate the cause of the Tubbs Fire due to its large size. The blaze was the most destructive — but not largest — in California history, burning 36,807 acres and destroying 5,643 structures.

"It's just a very complex fire," Mohler said. "It's not uncommon for a fire that size to take that long to investigate."

In May, CalFire said four other October fires were also caused by downed power lines owned by PG&E.

Mohler said the utility company has been "fully cooperative" in all of the investigations.

The findings could be especially devastating for PG&E, which has a history of being found responsible for major wildfires because of inadequate maintenance of its equipment.

The October fires have generated nearly $10 billion in insurance claims, according to the state, and resulted in numerous lawsuits filed against PG&E.



In addition to being held liable for fire costs, state utilities can also be fined by the California Public Utilities Commission, which regulates power companies.



Last year, the commission fined PG&E $8.3 million for failing to maintain a power line that ignited a fire in Amador County in 2015. That blaze, known as the Butte fire, burned 70,868 acres, destroyed 921 structures, and killed two people, according to CalFire.