The agency reported Wednesday that the Federal Mine Safety and Health Review Commission (FMSHRC) ruled in a hearing to resolve the dispute with the mine on October 2.
On October 16, administrative law judge Richard Manning ruled in favour of MSHA to uphold the citation, and required western Colorado mine Twentymile to provide a unit for storage of breathable air supplies.
"The vast majority of Twentymile Coal's emergency response plan was right on target; however, we took issue with the breathable air provision in the plan," said MSHA assistant secretary Richard Stickler. "Consequently, the case went before [the FMSHRC]."
Last month, the federal regulators said there was "reasonable possibility" workers at Twentymile's Foidel Creek operations could be trapped in main entries after an accident.
"In such a situation, miners likely would need an established facility to isolate them from hazardous mine environments and to provide breathable air so that they could survive until they could be rescued," the agency said.
"MSHA refused to fully approve the Foidel Creek ERP without a provision for providing breathable air for miners who may be trapped in the main entries."
The outlines of the Mine Improvement and New Emergency Response (MINER) Act of 2006 requires emergency and escape supplies to be provided at every US mine, as well as for the development and installation of ERPs. After a required federal review of all plans, MSHA found Twentymile's could not be approved due to the shortfall.
"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," Stickler said at the time.
The mine, located in Routt County, was the only underground coal mine not to have its plans approved. It remains unclear how much it will be fined.