The plan does not meet mandates because it does not include breathable air source locations, MSHA said in a statement to media. It is not clear how much the mine will be fined.

"Congress mandated that every underground coal mine have an approved emergency response plan in place to protect miners in the event of an accident," said agency assistant secretary Richard Stickler. "MSHA takes this requirement very seriously and will take action against any mine owner who fails to comply with the provisions of the complete emergency response plan."

Peabody responded in a statement to local newspaper the Craig Daily Press: "Twentymile believes it has submitted a very progressive plan and believes MSHA has approved similar plans at other operations," according to Derrell Carter.

"Twentymile continues to work with the agency via its formal review process to come to a resolution that meets our collective standards for the maximum safety of our employees."

A comment from the operator was not returned by press time.

A consistent top production performer, Twentymile is managed by Peabody's Foidel Creek Coal and is located in Routt County in the northwestern region of the state.