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A California utility said in a federal court filing this week that 2017's deadly Tubbs Fire in Napa, Sonoma and Lake counties, which killed 22 people, was likely sparked by electrical equipment on private property that had been installed and repaired without permits.

It was one of 22 devastating fires that spread across the wine country and beyond that fall, destroying more than 5,600 structures, state fire officials said. Pacific Gas and Electric has been blamed for a dozen of the October blazes, but the California Department of Forestry and Fire Protection, or Cal Fire, has not concluded its investigation into the cause of the Tubbs Fire.

"Our investigation is still ongoing," said Cal Fire spokesman Scott McLean. "I can’t address anything at this time until the investigation is complete."

A destroyed home in Santa Rosa, California, during the 2017 Tubbs Fire. Stephen Lam / Reuters

California Attorney General said in a court filing last week that PG&E could face charges up to murder if it is found criminally liable in any of the state's recent wildfires.

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PG&E, which serves 16 million people from Northern California to the state's Central Valley, said its equipment was not involved in the 36,000-acre Tubbs Fire, the state's second most destructive wildfire.

"The evidence supports the conclusion that this equipment, located beyond PG&E’s service delivery point, was planned, designed, installed, maintained and operated by third parties, not PG&E," lawyers for the utility wrote in a 16-page supplemental filing that offers their description of the incident.

A private property caretaker who had no electrical training and was not licensed to perform electrical work nonetheless had worked on the electrical gear, PG&E alleged in Monday's filing. Another man, also unlicensed, worked on electrical lines on the property, the utility said.

"Other third-party witness testimony suggests that other repair work on the private electrical equipment was previously required and performed without permits," the filing reads.

The supplemental document does not provide details about how the blaze was sparked, saying only that "when PG&E examined evidence at the incident location following the Tubbs Fire, PG&E observed that one of the customer-owned poles was severely burned at the top."

"We have also shared the same information and data with Cal Fire," PG&E spokesman James Noonan said in an email.

An aerial view of homes that were destroyed by the Tubbs Fire in 2017 in Santa Rosa, California. Justin Sullivan / Getty Images

The information is part of a 390-page court filing to U.S. District Judge William Alsup describing what if any role PG&E played in starting a series of wildfires in the fall of 2017, as well as 2018's Camp Fire.

The judge wants to determine whether the utility violated the terms of its agreement with the government on the deadly San Bruno gas pipeline explosion.

The Camp Fire tore through Butte County in November, killing 88 people, torching 153,336 acres and destroying nearly 14,000 homes.

Federal prosecutors first filed suit in 2014 against PG&E in connection with the 2010 explosion; the case was adjudicated in 2017.