Oregon officials have added the state’s largest natural gas utility as a defendant in their $50 billion lawsuit against fossil fuel companies over their contribution to climate change.

The suit — the first to make climate-related deception claims against a utility, experts said — alleges that the company, NW Natural, knew that the burning of natural gas contributed to global warming, but misled its customers about the consequences. It is an expansion of a lawsuit that was first filed last year by Multnomah County, which includes Portland.

The complaint alleges that Exxon, Shell and other companies, now including NW Natural, had schemed to “rapaciously” sell fossil fuel products and cover up what they knew about the risks to the planet, and that the companies bear responsibility for the effects of climate change including a deadly heat wave in 2021 that killed at least 69 county residents.

Experts said the addition of NW Natural to the amended lawsuit on Monday was the first instance of a gas utility being named as a plaintiff in a case like this. Overall there are about two dozen lawsuits by state and local governments alleging that oil companies covered up the dangers of climate change.

Alyssa Johl, vice president of legal and general counsel at the nonprofit advocacy group the Center for Climate Integrity, said that gas utilities had been “significant players in the historic and ongoing deception campaigns to mislead the public about the dangers of fossil fuels.”

NW Natural said Wednesday that it had not received the complaint, though it was aware of reports that it had been added to the lawsuit. “We believe adding the company to the suit now is an attempt to divert attention from legal and factual flaws in the case. NW Natural will vigorously contest the County’s claims should they come to court,” the company said in a statement.

The original suit also named other companies and industry trade groups as defendants, including the consulting giant McKinsey & Company. In a statement, Exxon Mobil vigorously disputed the claims and said it was investing more than $20 billion in “lower-emission initiatives” through 2027.

“The baseless claims made in these politically motivated lawsuits do not advance any real solutions to address climate change,” Exxon Mobil said. “We’ll continue to fight these lawsuits and claims, and, more importantly, continue to be a leader in the energy transition.”

Shell declined to comment. McKinsey did not immediately respond to a request for comment.

The lawsuit alleges that the defendants had known about the risks of global warming since the 1950s because of their own internal forecasting, and covered up that information. The county said it had incurred more than $50 million in damages from wildfires and extreme heat and other disasters, and that future economic damage would amount to at least $1.5 billion. The suit seeks a much greater sum, “at least $50 billion,” for adaptation projects to forestall harms related to climate change.

The amended complaint also added the Oregon Institute of Science and Medicine, a nonprofit research group, as a defendant, alleging that it ran a misinformation campaign with funding from Exxon Mobil, among others. An email to the address listed on the organization’s website on Wednesday bounced back.

The chair of Multnomah County, Jessica Vega Pederson, cited the lawsuits against tobacco companies in the 1990s that led to extraordinary payouts. Lawyers have looked to those cases, as well as litigation over the opioid crisis, as blueprints for action against oil companies over climate change.

“We’re already paying dearly in Multnomah County for our climate crisis with our tax dollars, with our health and with our lives,” Ms. Vega Pederson said.

The attorneys general of 10 states or territories have sued oil companies over what they say is deception about the dangers of climate change. Municipal and tribal governments have filed similar suits. None has gone to trial yet, amid years of wrangling over whether they belong in state court (where they were filed, and where legal experts think the plaintiffs will fare better) or federal courts (where fossil fuel companies argue they belong).

In June a federal judge rejected the defendants’ attempts to get the Multnomah County case moved to federal court.

Legal experts say the case in Massachusetts is likely to be the first to make it to trial, as early as next year. That case was filed in 2019 by then-Attorney General Maura Healey against Exxon Mobil, alleging that the company had violated consumer protection laws by misleading customers and investors.

The climate change lawsuits, which have mostly come from Democratic-led states, are also being challenged by the governments of Republican-led states. In May, Alabama and 18 other states asked the Supreme Court to block climate-related lawsuits by California, Connecticut, Minnesota, New Jersey and Rhode Island against oil and gas companies.

The plaintiffs argued that the defendants were trying to “dictate the future of the American energy industry” by imposing “ruinous liability and coercive remedies” on companies. They also argued that disputes over emissions that cross state lines fall under federal, not state law.

On Monday, the Supreme Court justices asked the U.S. solicitor general, Elizabeth Prelogar, for her input on the case, in a sign that they could decide to hear the case.