Ouray County officials said they have ruled out an explosion and mine collapse as the immediate cause of the deaths of Nick Cappanno, 34, of Montrose and Rick Williams, 59, of Durango on Sunday at the Revenue-Virginius mine.
However, the Montrose Daily Press and The Denver Post reported that investigators were looking at whether a small blast on Saturday at the underground gold, silver and lead mine may have been the cause of the carbon monoxide problem.
"I believe a blast was involved which did create some gases which potentially the miners did inhale," Rory Williams, manager for the mine's owner, Denver-based Star Mine Operations LLC, told the Daily Press.
Nineteen other miners were taken to a hospital and four were admitted. They were listed in fair condition.
The investigation will be done by the U.S. Mine Safety and Health Administration.
A possible blast on Saturday would be something investigators would look into as part of the chronology of events leading to the deaths, administration spokeswoman Amy Louviere said.
Officials also will interview witnesses and inspect any equipment involved for possible malfunctions, she said.
Rory Williams, who is not related to Rick Williams, said all of the men are required to wear personal respirators and the two who died had them. It was unclear, however, if the workers were wearing the respirators.
Rory Williams said it doesn't appear there was an equipment malfunction, the Post reported.
"I knew both of these individuals personally," Rory Williams said. "They were hard-working men. They were great men. They will be remembered indeed."
About 100 miners work at the site about 270 miles southwest of Denver, which will be closed until the end of the investigation.
Star Mine acquired the property in 2011 and got a permit to start mining in February. All the mining is done by drilling and blasting, according to state regulators.
The last major mining disaster in Colorado occurred on April 15, 1981, when an explosion killed 15 people at the Mid-Continent Dutch Creek No. 1 Mine near Redstone.
There have been eight mining deaths in the state since 2002, not including the two Sunday, according to the mine safety agency.
In 2011, a New Mexico contract worker died after being hurt at the West Elk Coal Mine in Somerset, in western Colorado. The agency found the 53-year-old worker slipped and fell from a beam at a tower construction site.
In 2012, a 25-year-old water truck driver died after losing control of his vehicle at Colowyo Mine in Moffat County.