The United States’ third-biggest remaining coal miner by volume, Gillette, Wyoming-based Cloud Peak Energy, has filed for bankruptcy.

The company owns three mines in the Powder River Basin in Wyoming and Montana. The Associated Press says the bankruptcy was prompted by U.S. utilities switching their operations from coal to natural gas.

“While we undertake this process, Cloud Peak Energy remains a reliable source of high-quality coal for customers,” President and CEO Colin Marshall said in a release.

AP says the bankruptcy had been expected, after Cloud Peak missed a deadline for a US$1.8-million loan payment in March. The last of several extensions on that deadline expired Friday. It’s the fourth company in recent years to file for bankruptcy in the country’s top coal-mining state.

The mines, which shipped 50 million tons of coal last year, “will remain in operation during the bankruptcy process with help from $35 million in debtor-in-possession financing,” the news agency states. Wyoming officials have “sought to encourage carbon-capture technology by funding research at a Gillette-area power plant,” but “any marketable results from that work remain years away from wide-scale use.”