Mining remains on hold at the underground Signal Peak coal mine south of Roundup as workers attempt to lower concentrations of a dangerous gas in a sealed-off area.
Amy Louviere with the federal Mine Safety and Health Administration said agency engineers are due at the mine near Roundup on Thursday to evaluate its ventilation system.
Since last week, workers have been pumping nitrogen into a previously-mined area where high levels of carbon monoxide were found, prompting the removal of about 45 workers.
Signal Peak spokesman Mike Dawson says work above ground is continuing and no layoffs are expected among the mine's 300 workers. He says there is no projection for when mining will resume.