WELLINGTON, Utah -- Arch Coal Co. says it has resumed full operation of Utah's Dugout Canyon Mine after months of problems with elevated carbon monoxide levels.
The company says it isn't clear what caused the spikes for brief intervals over the past four months, keeping the mine largely shut down.
Federal regulators say a coal seam started smoldering last March, but an Arch spokeswoman told The Associated Press it was a flash ignition that almost immediately died.
In June, carbon monoxide levels spiked again for reasons the company says aren't clear, shutting down the mine again.
Arch Coal says it resumed mining Aug. 17 using portable diggers called continuous miners, then restarted a longwall mining machine on Tuesday.
That brought the mine back into full operation.