Twenty other workers were injured in the accident near the southwestern Colorado town of Ouray.
The Denver Post reports that officials said at a nighttime news conference that the source of the gas was under investigation.
They are looking at whether a small explosion in the mining process might have caused it.
Ouray County spokeswoman Marti Whitmore says 20 people were taken to area hospitals, and all but two have been treated and released.
The Montrose Daily Press reports that 10 of them went to Montrose Memorial Hospital, where officials say they were treated for carbon monoxide exposure.
Rory Williams, the operations manager for Denver-based Star Mine LLC, says all of the men are required to wear personal respirators and the two who died had them. He says that it doesn’t appear to be an equipment malfunction.
Source: The Associated Press