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A dozen of the wind-driven blazes that swept northern California's wine country last fall, killing 46 people in the deadliest firestorm in state history, were sparked by PG&E-owned power lines, state officials said on Friday (local time).
The California Department of Forestry and Fire Protection, or CalFire, also said its investigators had found "evidence of alleged violations of state law" by Pacific Gas & Electric Company and referred those cases to county prosecutors for further review.
PG&E issued a statement in response saying the company looked forward to reviewing the CalFire reports, adding, "We continue to believe our overall programs met our state's high standards."
The findings could have tremendous implications for the San Francisco-based utility company in terms of potential legal liability for one of California's most lethal and costly disasters.
PG&E shares were down 0.6 percent in after-market trading following the CalFire statement.
The agency's investigation covered 12 individual blazes accounting for much of the conflagrations that erupted the night of October 8 and raced across several counties north of San Francisco, collectively dubbed the North Bay Fires, or the October 2017 Fire Siege.
The firestorm claimed 46 lives, scorched at least 99,148 hectares, and incinerated 8900 homes and other structures, including entire subdivisions in the Sonoma County town of Santa Rosa. An estimated 100,000 people were placed under evacuation orders and the region's renowned wine-making region was thrown into turmoil.
The state's insurance commissioner put insured losses from the tragedy at $US9 billion.
Causes cited by CalFire included downed power lines and trees blown into electrical wires -- all equipment owned by PG&E -- during the course of fierce winds that roared through the region to spread the resulting flames with unprecedented speed.
Code violations were alleged in eight of the 12 fires under examination, CalFire said.
A CalFire spokesman, Michael Mohler, said agency officials looked for specific lapses ranging from maintenance and equipment failures to improper vegetation clearance, but he declined to discuss specific findings because the matter remains under investigation.
Last month, the agency similarly issued findings for the first four fires it examined from the North Bay incident, concluding those, too, were triggered by trees falling into PG&E power lines. Alleged clearance violations were cited in three of those instances. Mohler said investigations were still under way for several remaining fires from last October's outbreak.
PG&E said it meets or exceeds regulatory requirements for maintaining its utility poles, inspecting every electric transmission and distribution line at least once a year and pruning or removing some 1.4 million trees annually.
The company cited the "new normal" of heightened year-round wildfire hazards created by years of prolonged drought and millions of dead trees.
In the case of last year's disaster, PG&E also pointed to the confluence of an unusually wet winter spurring heavy growth in vegetation, which dried out in record-setting summer heat to set the stage for explosive fire activity when a high winds struck that fall.