Two miners died from what is believed to be carbon monoxide poisoning and 20 more were sent to hospitals in Colorado’s Western Slope region on Nov. 17.

The incident occurred at the Revenue-Virginius silver and gold mine, 10 km (6 miles) south of Ouray, CO.

According to a report in the Denver Post, an explosion was ruled out as the immediate cause of death to the miners. The source of the gas was under investigation as of Monday, Nov. 18.

"Anything that has been reported is speculative," said Marti Whitmore, a spokesperson for the state's emergency management office. "We don't know what the cause is." Officials said they were investigating whether a small explosion in the mining process might have caused the release of the gas.

The historic mine resumed operations this year. It received a permit from the state Feb. 5 to mine silver, gold and. According to state regulators, all mining takes place below ground using drill-and-blast methods.

The miners who were killed were identified as 34-year-old Nick Cappano of Montrose, CO and Rick Williams, 59, of Durango, CO. The other miners were expected to recover, said Rory Williams, the operations manager for Denver-based Star Mine LLC.

“I knew both of these individuals personally,” said Williams, who said he is no relation to Rick Williams. “They were hard-working men. They were great men. They will be remembered indeed.”

Williams said all of the men are required to wear personal respirators and the two who died had them.

“As far as we can tell it doesn't appear to be an equipment malfunction,” he said.

Crews worked around the clock to stabilize the mine's tunnels to allow work to resume, The Watch newspaper in Ouray reported.

The Colorado Division of Reclamation Mining and Safety issued a statement saying the agency would work with U.S. Mine Safety and Health Administration (MSHA) on the investigation.

"The Division of Reclamation Mining and Safety extends its deepest sympathies to the miners and families of those affected by this tragedy," the agency stated.

The incident was reported at 7:20 a.m. Nov. 17. All but the two dead miners made it out on their own. A rescue party reached the bodies at about 3 p.m.

Before Sunday, the last mining fatality in Colorado happened July 14, 2012, when a water truck that 25-year-old Jason Kawcak was driving overturned at the Colowyo Mine in Moffat County, according to the U.S. Department of Labor.

There have been 36 fatalities at U.S. mines this year, according to the federal Department of Labor. There have been 15 mining deaths in Colorado in the past decade, according to MSHA.