The falls caused damage to some of the mine’s ventilation controls in areas already mined. The Pennsylvania-based operator said two monitoring stations were showing Friday that carbon monoxide levels were above ambient levels but continuing to drop.

“The source of the carbon monoxide has not been determined,” Consol said.

“Engineers theorise that it may be residual gas remaining in inactive mine areas from a fire that occurred in the mine in 2005, or it could be from an ignition of a small amount of methane caused by one of the roof falls earlier this week.”

In the meantime, the operation remains idled while the workings are ventilated. Customers are receiving coal shipments from surface stockpiles.

Meanwhile, Buchanan engineers have submitted borehole drilling plans to the state and federal regulators so that additional monitoring points can be developed. Additionally, Consol said, those boreholes could be used to insert inert gas or water if the need arises.

“Engineers believe that this action may flush residual amounts of carbon monoxide into the active ventilation flow so that the gas will be swept from the mine,” it said.

“Once all monitoring stations show acceptable levels of the gases being monitored, and agency approval is secured, teams can re-enter the mine.”

No injuries occurred in the roof falls at Buchanan, which is located near Mavisdale in the southwest region of the state.