Sagging power lines owned by Pacific Gas and Electric Co. (PG&E) caused a massive wildfire in Northern California last year that killed four people and injured a firefighter, an investigation by the California Department of Forestry and Fire Protection (CAL FIRE) has determined.

The Cascade Fire in Yuba County last October was started when the sagging power lines came into contact during heavy winds, according to the report, which noted that no violations of the public resource code were found by the investigators. Fire officials commonly refer to the cause as “line slap.”

CAL FIRE said the report was sent to the Yuba County District Attorney’s Office.

PG&E could not be reached to comment. Spokeswoman Lynsey Paulo, however, told the Associated Press that the utility recognizes it must do more to help lower the risk of fires in the future. PG&E and other utilities are strengthening poles and lines, upgrading weather modeling and clearing vegetation around equipment.

When swaying power lines made contact during heavy winds, it caused an electrical arc that ignited the fire on Oct. 8, 2017, burning nearly 10,000 acres, destroying 264 structures and causing the fatalities and injuries. “The electrical arc deposited hot burning or molten material onto the ground in a receptive fuel bed causing the fire,” a CAL FIRE spokesperson said.

CAL FIRE investigators were dispatched to more than a dozen wildfires that ravaged historic wine country and other areas of Northern California. They have determined what caused most of the fires but some work continues, including for the Tubbs Fire, which was the most destructive in state history.

The October 2017 fire siege involved more than 170 fires and burned at least 245,000 acres in Northern California. Approximately 11,000 firefighters from 17 states and Australia helped battle the blazes. The fires left 44 people dead and more than 5,000 homes destroyed. The state estimates that insured damages exceed $9 billion.

Fire investigators collectively have blamed PG&E equipment for at least 12 of those blazes, including two that killed 15 people. In the wake of the fire in both Northern and Southern California, the state’s major investor-owned utilities are financially vulnerable, a situation that was partially addressed by a series of new state laws signed by the governor last month.