State Department of Environmental Quality officials say they need more information from St. Louis-based Arch Coal, Inc. to complete their environmental study of the proposed Otter Creek coal mine.
The study must be finished before Arch can get a permit to mine an estimated 1.4 billion tons of coal near the Wyoming border.
Under a recent agreement between the state and Arch, the permit decision date was bumped from August 2013 to late 2015.
It could take years more to develop the mine and a rail spur needed to ship the fuel out of Montana.
An Arch spokeswoman says the company plans to submit a revised permit application later this year.