A county in Oregon has expanded its ambitious $51.5 billion climate lawsuit by naming the state’s largest natural gas utility, NW Natural, as a defendant.

The legal move targets fossil fuel companies over their alleged contributions to the deadly 2021 heat dome event.

First filed last year, the lawsuit from Multnomah County, which includes Portland, claims that carbon emissions from the companies were a major factor in the heat dome that set temperature records across the Pacific Northwest.

The extreme heatwave, occurring between late June and early July 2021, resulted in some 800 deaths in Oregon, Washington, and British Columbia.

The amended complaint introduced this week brings NW Natural into a case that already lists prominent oil firms such as ExxonMobil, Chevron, and Shell as defendants, the Assocated Press reports. It accuses NW Natural, which serves about two million customers in the Pacific Northwest, of being responsible for “a substantial portion” of Oregon’s greenhouse gas emissions and alleged misinformation regarding the impact of these emissions.

NW Natural has refrained from detailed commentary while it examines the claims. In an emailed statement, though, the company described the new allegations as an effort to divert attention from what it sees as legal and factual shortcomings in the case. “NW Natural will vigorously contest the County’s claims should they come to court,” the company said, the AP reported.

The Center for Climate Integrity notes that this is a landmark moment, as it marks the first instance of a gas utility being implicated in a lawsuit accusing fossil fuel companies of climate deception. Currently, more than two dozen similar lawsuits are ongoing, initiated by state, local, and tribal governments across America.

Multnomah County seeks damages primarily to cover the expected costs of dealing with extreme weather events such as heatwaves, wildfires, and droughts. “We’re already paying dearly in Multnomah County for our climate crisis — with our tax dollars, with our health and with our lives,” the county chairwoman, Jessica Vega Pederson, told the wire service. “Going forward we have to strengthen our safety net just to keep people safe.”