SAN FRANCISCO — Investigators are looking into whether fires ravaging Northern California may have been sparked by Pacific Gas & Electric equipment, deepening skepticism around the utility’s controversial plan to prevent fires in the region by shutting off power to millions of residents — and raising panic among the bankrupt company’s investors.

PG&E told regulators last week that equipment on one of its transmission towers broke near the ignition point of the Kincade Fire, a blaze that has destroyed more than 120 structures and burned 75,415 acres of Sonoma County since last Wednesday, making it the state’s biggest fire this year. The utility also said its equipment was found damaged near the site of two smaller fires over the weekend, one of which burned a tennis club to the ground.

The fires have deepened the sense of frustration felt by local residents and politicians as California’s largest utility company embarks on an unprecedented schedule of forced blackouts. Buried in a contentious bankruptcy proceeding, unable to pay victims of past fires and preparing for the danger of hurricane-like winds, PG&E tried to stave off more damage and future liability by turning off power to more than 2 million customers. PG&E serves about 16 million customers over 70,000 square miles in Northern and Central California.

The blackouts may have helped contain the destruction; as of Tuesday, no deaths had been reported in connection to the fires, according to the California Department of Forestry and Fire Protection.

But the blowback from residents who have been forced to live on generators and batteries, and the growing concern that the power shut-offs have failed to prevent the exact problem they were designed to solve, have plunged PG&E into deeper political and financial crisis.

“I think they’ve been supporting their shareholders and executives more than they’ve been looking out for the public,” said Randy Harris, the owner of a Santa Rosa-based gift shop that has lost customers because of the blackouts.

Northern California is bracing for another windstorm that could complicate efforts to contain the Kincade Fire, which has forced nearly 200,000 wine country residents to evacuate and is moving steadily toward the city of Santa Rosa. The National Weather Service issued a wind advisory for parts of Sonoma and Napa counties between noon Tuesday and 11 a.m. Wednesday, with the strongest gusts expected overnight. The increased wind will create “rapid fire growth potential,” the Weather Service said.

Gov. Gavin Newsom (D) stepped up his criticism of PG&E in a news conference Monday, calling PG&E’s plan to implement blackouts over the next 10 years “unacceptable” and publicly soliciting Warren Buffett’s Berkshire Hathaway to make a takeover offer to bail out the embattled utility company.

PG&E’s stock price has plummeted more than 40 percent in the past week as evidence began pointing to the utility company’s aging infrastructure as a possible cause of the Sonoma fire. Citi analyst Praful Mehta predicted the company’s equity value could soon fall to zero, as the new fires have created the potential for even more liabilities the company can’t afford to pay.

“It’s a lose-lose situation right now for the utility,” Mehta said in an interview. “They switch off power; everybody’s upset. If they don’t switch off the power, and there are fires, everybody is still upset.”

With the pool of investors willing to support PG&E shrinking, the bankrupt utility is not in a position to address the underlying problems with its aging infrastructure, which would require many years, billions of dollars and innovative steps such as moving power lines underground, said Michael Wara, director of the Climate and Energy Policy Program at Stanford Law School.

“Someone has to invest,” Wara said. “Those people will have to be excited about loaning money to PG&E. Those people have to have a perception that PG&E is a low-risk investment — it’s not.”

This predicament is increasing the likelihood of a public takeover of the utility, Wara said. “You may have to hitch it to something that can borrow money or give it a decent credit rating.” The utility’s largest remaining shareholders include hedge funds Abrams, Baupost, Knighthead and Appaloosa.

On Tuesday, PG&E began its third series of power shut-offs, expected to affect 600,000 customers across the Bay Area.

“Our sole purpose of implementing power shut-offs is to reduce the risk of catastrophic wildfire,” said Ari Vanrenen, a PG&E spokeswoman. “We recognize the hardship of not having electric service and what that puts on our customers."

PG&E says it had shut off electricity to the local distribution lines in Sonoma County at the time the Kincade Fire started. But electricity still flowed through what are called transmission lines, which carry power over a larger region, according to Ian Hiskens, a University of Michigan engineering professor who specializes in power system dynamics and grid controllability.

“They’re the freeways of the electricity system,” Hiskens said of the transmission lines.

The major transmission routes — which involve higher wiring and better clearing — can’t be cut without even larger disruptions. If the transmission lines were to be shut down, power routed from energy suppliers in Oregon and the Pacific Northwest wouldn’t make its way through California to locations as far south as Los Angeles, Hiskens said.

The fires that started on Sunday in the suburbs east of San Francisco fell just outside PG&E’s blackout region, Vanrenen said. That illustrated the “ongoing, dynamic” nature of the high winds, which make it difficult to predict what areas are prone to fire, she added.

PG&E’s power shut-off plan follows a similar approach to wildfire prevention taken by San Diego Gas & Electric over the past decade. Following 2007 fires that burned hundreds of homes in the city and killed two residents, SDP&E began using “surgical” shut-offs to its electrical grid to prevent fires, said Katie Bays, co-founder of research and consulting firm Sandhill Strategy.

The approach taken by PG&E, by contrast, “is like taking a sledgehammer to the grid,” Bays said.

Experts who study energy and the power grid said reoccurring planned shut-offs of this scale are unprecedented. Developing countries conduct targeted power shut-offs for hours at a time to prevent an overloading of the power supply, but days-long outages aren’t the norm even in countries with weak power grids, Hiskens said.

“In India, for example, everyone knows the power’s going to go out, so they have their backup generator,” he said. “Their lifestyle is built around losing their power regularly. In first-world countries, when the power goes out, people are like, ‘What the heck just happened?’ and within a short period of time the fridges are not that cold anymore.”

In areas of California where power shut-offs were beginning to hit more regularly, they have begun to affect businesses and vulnerable populations who rely on electricity to survive.

In a letter to Newsom this week, U.S. Veterans Affairs Secretary Robert Wilkie raised “serious concerns” that the PG&E blackouts could endanger the veterans in the region who require regular care.

“Power outages could become dangerous for veterans receiving in-home care and those who rely on power for the refrigeration of life saving medications like insulin,” Wilkie said in the letter.

Tyler Radak, 29, who lives in Moraga, Calif. with her husband and their 18-month-old daughter, said they were spending nights huddled around a gas fireplace to stay warm, with the baby wrapped in two layers of fleece. This weekend, with an electric stove unusable, they heated water and grilled food using a propane barbecue outside.

The shut-offs, said Radak, entail “trying to cook before everything goes off, trying to get laundry done before it all goes off, trying to plan ahead and have things on the counter that don’t need to be refrigerated so I can try to feed my daughter.” She said when the power turns back on they charge up a supply of portable batteries to keep their devices working.

The family has their documents packed away in case they need to leave, and they’re stashing supplies such as diapers.

She had a simple message for PG&E: “Find the budget to bury the power lines.”

Heather Kelly contributed to this story.