When Gunvor, one of the world's largest energy traders, invested $400 million in a troubled Montana coal mine, it looked like a promising deal. The plan was to sell much of the coal to the booming Asian market, where it was fetching far more than the prevailing price in the U.S.

The deal in October 2011 delivered impressive profits to the mine's previous owners, a rare financial success in America's depressed coal industry. It also set the mine on course to more than double production this year compared with last, at a time when total U.S. coal production is falling.

But shipping coal to the Asia Pacific region was not as straightforward as it seemed. Gunvor expanded from its core of trading oil into other energy sectors just as the price of coal in the U.S., Asia and elsewhere tumbled in part because U.S. power plants are burning cheaper natural gas. Now, the Geneva-based company is battling unexpected economic, political, and legal headwinds in the U.S.

Gunvor and the two other owners of the Montana mine, called Signal Peak, are embroiled in a legal dispute over royalty payments. The mine has also bid on federal and state coal tracts, thrusting Gunvor into a debate raging in America over whether governments are getting a fair price for U.S. coal reserves.

Industry experts say Gunvor bought the mine at the height of the market. "The timing of the deal was terrible," said a senior coal trader with a rival trading house. "They overpaid."

In a statement last month, Gunvor defended "Signal Peak's long-term value," noting both the quality of the coal and the quantity — enough for the mine to keep producing for roughly 30 years.