A private electrical system -- not equipment owned by embattled utility PG&E -- caused a 2017 wildfire in Northern California wine country that destroyed thousands of homes and killed 22 people, state investigators said Thursday.

The Tubbs fire, which destroyed more than 5,600 structures over more than 57 square miles in Sonoma and Napa counties, was one of more than 170 that torched the state in October 2017. It was the most destructive wildfire in California's history before the Camp Fire, which destroyed more than 239 square miles, nearly 19,000 buildings and killed 86 people this past November.

California's firefighting agency officially ascribed the fire to "an unknown event affecting privately owned conductor or equipment." Some details about the property, including its owner and address, were blacked out in the report. It said the Napa County property about 3 miles north of Calistoga was built in 1946 on about 10.5 acres with a wine cellar, pool and several outbuildings.

PG&E said in a Jan. 2 court filing that it believed a handyman performing unlicensed electrical work started the fire. In that filing, it identified the owner of the Napa County compound as Ann Zink. The utility said it provided electricity to Zink's property by a line that connected to a service riser but that Zink had a private system to carry power to other buildings as well as equipment such as a water pump and water storage tank.

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PG&E said it had no responsibility to maintain or inspect the private system.

Zink, 91, told The San Francisco Chronicle in 2017 that her house was unoccupied at the time of the fire and she was at her other home in Riverside County when the blaze began.

PG&E previously said it plans to file for bankruptcy protection next week, citing billions of dollars in potential damages from lawsuits linking its equipment to other deadly blazes for which it has been determined to be at fault.

The company said in a statement that despite Thursday's finding, PG&E "still faces extensive litigation, significant potential liabilities and a deteriorating financial situation."

Gov. Gavin Newson said it's up to PG&E to decide whether to move ahead with a planned bankruptcy given that more than half of its expected damages stemmed from the Tubbs Fire.

He said his goal is not to rescue PG&E but to make sure victims are made whole, that the state has "safe, reliable and affordable service" and that rate payers "are not paying the price of the neglect" that has been established in past wildfires.

Newsom also said he doubts the report will end litigation related to the wildfire.

Michael Kelly, an attorney for victims of the fire, said the findings wouldn't have much effect on the lawsuits he has filed.

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"We're going to stick by our guns," Kelly said, adding that there are still questions about why PG&E didn't cut power to the area despite a high fire danger. He said there is also evidence that contradicts the findings of state fire investigators.

Trading of PG&E Corp. stock was halted twice after news about the cause of the fire prompted a surge of buy orders. Once trading resumed, the price rocketed up, closing up $5.96, or nearly 75 percent, at $13.35 a share.

A state senator said that just because a private electric line caused the wine country fire does not let the utility off the hook for the role of its equipment in other devastating fires in the state.

State Sen. Bill Dodd, a Napa Democrat, cited system-wide issues plaguing California's largest utility.

Lawmakers are under pressure to find a solution that addresses utility reform and compensates wildfire victims.

"This underscores the idea that we all have a role to play in wildfire prevention," said Dodd a frequent critic of PG&E, who noted that the company has already been found at fault for more than a dozen other Northern California wildfires.

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In the report released Thursday by the state, one witness reported seeing a transformer explode. Another reported seeing the fire approach a PG&E power pole.

One witness, Charlie Brown Jr. of Calistoga, said the electrical wiring leading from the property where investigators concluded the fire started had not been used in years.

The Associated Press contributed to this report.