The most devastating of the 2017 Wine Country fires, the second-most destructive blaze in California history, was sparked by private electrical equipment, state officials reported Thursday.

The Tubbs Fire killed 24 people and destroyed more than 5,600 buildings when it raced from rural Napa County to suburban Santa Rosa, becoming an enduring symbol of the state’s increasing wildfire risk. It stands out from many of the blazes that erupted in fall 2017 because it wasn’t blamed on Pacific Gas and Electric Co.

An 80-page report by the California Department of Forestry and Fire Prevention, Cal Fire, faults a privately owned “conductor or equipment” at a home in Calistoga, or next to it, for the blaze. The document doesn’t narrow the ignition to a precise location within the parcel’s electrical setup, nor identify exactly what went wrong, however. The fire destroyed much of the wooded 10.5-acre lot, making an already difficult investigative task even trickier.

The report, which has been in the works for 15 months, suggests that at least parts of the property’s electrical system were not “professionally installed.” A caretaker interviewed by investigators acknowledged that repairs were needed, including replacement of a wooden electrical pole that was “wood-peckered so damn bad,” though there was no evidence this caused the blaze. The report also says two fire-safety violations had been found on the parcel during a 2015 defensible-space inspection.

Still, investigators concluded that there were no breaches of state law associated with the electrical issues that ignited the Tubbs Fire, and no criminal charges were recommended: “No negligence, in that matter of speaking,” said Cal Fire spokesman Scott McLean.

The owner of the property, Ann Zink, 91, could not immediately be reached for comment, nor could the caretaker. In 2017, Zink told The Chronicle that her Calistoga home was unoccupied at the time of the fire, but she thought trees hitting wires during fierce winds started the blaze. She said she was at her other house in Riverside County when the fire broke out.

PG&E, which has been blamed for more than a dozen of the October wildfires, has long argued that private electrical work at the home was responsible for the blaze.

Within days of the Oct. 8 ignition, Cal Fire investigators had taped off several properties around Zink’s home on Calistoga’s Bennett Lane, east of Highway 128, an area where forested hillsides converge on leafy vineyards. The site was treated as a crime scene for weeks.

Tubbs Fire: Follow the blaze's spread

After lengthy examination of electrical equipment and burn patterns as well as consultation with witnesses and fire experts, investigators ruled out the possibility that PG&E caused the fire as well as the likelihood of arson. A hitchhiker known to local authorities was investigated briefly, according to the report.

Those who lost homes, even friends and family, in the Tubbs Fire had mixed reaction to the investigation’s findings. Most were surprised, however, to learn that PG&E wasn’t the culprit.

“How?” said Himanshu Parikh, 51, confused by the news. “Did they prove it? How? Wow.”

Like many in Sonoma and Napa counties, Parikh, whose house burned in Santa Rosa’s Fountaingrove neighborhood, said he was banking on a legal payout from PG&E. He said he needs help covering his rent until his home is rebuilt. He said he also doesn’t expect his insurance to fully cover the cost of his new house.

“We have heard ‘no’ from so many other places, like FEMA and insurance,” said Parikh, noting that this was his first home in the United States, having moved from India in 2008 with his family, including his now-15-year-old daughter and 21-year-old son. “This is very bad for us.”

PG&E’s vindication, though, appeared to be good news for Wall Street. Shares of the utility’s parent company, PG&E Corp, shot up nearly 75 percent Thursday, even as the company announced earlier this month that it planned to file for bankruptcy because of growing fire liabilities.

The Tubbs Fire was among at least 170 separate ignitions that occurred across Northern California during that second week of October. Many became violent blazes, including the North Bay’s Atlas and Nuns fires nearby. Between the Mendocino coast and the Sierra foothills, a total of 45 people were killed and more than 9,000 homes, businesses and other structures were destroyed.

Wine Country was ground zero of the unprecedented firestorm. The Tubbs Fire was by far the most catastrophic.

The blaze was reported to fire officials at about 9:45 p.m. during an evening of warm temperatures and gusts up to 80 mph. What started as a small fire on a country lane quickly escalated into an inferno that shot through the forested hills and canyons of Napa and Sonoma counties before descending toward the more populous Santa Rosa.

By 1 a.m., homes were burning along Mark West Springs Road, just north of the city, and in an early-morning panic, thousands of unsuspecting residents began scampering for safety, joining bumper-to-bumper traffic on the region’s few major roads. Some, however, became trapped in their homes and in a few cases jumped into swimming pools to escape the flames.

The swift winds carried flames across Highway 101 and into the sprawling Coffey Park subdivision, where hundreds of tract homes burned to the ground. As the fire hopscotched across Santa Rosa, the exclusive Fountaingrove neighborhood was decimated, as were schools, health centers and businesses, including a Kmart and a Hilton hotel. Also hit hard was the Larkfield-Wikiup community.

Firefighters were left dumbfounded by the intensity of the burn. A total of 36,807 acres were left in ruin.

The blaze has been held up as a demonstration of the worsening effects of wildfire and as a teachable moment for fire prevention and safety. The region’s emergency warning system, for example, faced a crucial test that it failed, state officials said later, giving rise to changes in statewide communication protocols.

Most of those who lost homes and commercial property are just beginning to rebuild. Many have left the area. Insurance estimates put statewide losses above $11 billion.

For Santa Rosa resident Janet Reisner, 59, knowing the cause of the blaze that destroyed so much might have brought a sense of closure. But it didn’t, she said.

In the beginning, at least, there had been a massive display of public support, with businesses hanging “Santa Rosa Strong” signs in their windows and donations pouring in from across the country, Reisner said. But over the past year, the attention dropped off, focused on other fire-scarred places like Redding and Paradise.

Meanwhile, the 15 months since the Tubbs Fire leveled her Coffey Park home has felt never-ending, she said.

“It’s probably worse now than the beginning ever was, because it’s been so slow rebuilding,” said Reisner, who works for the city. “It’s been really tough on my husband and myself. We are still living in a rental.”

The tiny bit of hope for the Reisners was that some money might come from PG&E, if the utility was found liable. Thursday’s report left them feeling disappointed and shocked, and unable to fully process the news.

Cal Fire has blamed PG&E for 17 of the wildfires in October 2017. In 11 of the cases, investigators said the utility broke a law. They have not specified the violation, saying matters are still being investigated for criminal prosecution, but presumably the company failed to safely maintain electrical equipment.

While PG&E may be cleared of culpability in the Tubbs Fire, the utility is still facing hundreds of lawsuits and as much as $30 billion worth of claims over damages wrought by fires over the past two years.

Investigators are still looking into the cause of November’s 153,000-acre Camp Fire, which surpassed the Tubbs Fire as the most destructive in state history. The blaze killed 86 people and destroyed nearly 19,000 structures, virtually wiping out the community of Paradise in the northern Sierra foothills.

PG&E reported that a high-voltage line malfunctioned near the origin of the Camp Fire, which has since fanned both speculation and legal complaints alleging that the utility was behind the disaster.

Chronicle staff writer Peter Fimrite contributed to this report.

Kurtis Alexander, Evan Sernoffsky and Lizzie Johnson are San Francisco Chronicle staff writers. Email: kalexander@sfchronicle.com, esernoffsky@sfchronicle.com, ljohnson@sfchronicle.com Twitter: @kurtisalexander @EvanSernoffsky @lizziejohnsonnn