According to the Mine Safety and Health Administration, a foreman and a miner at the Revenue Virginius Mine, which conducts underground gold and silver mining, were overcome by gas in an area where an explosive had been detonated.

Officials said, the fallen miners, identified as Nick Cappanno, 34, of Montrose, and Rick Williams, 59, of Durango, died of carbon monoxide poisoning. Mine rescue teams searching for the men detected fatal levels of the gas, and 20 miners were taken to hospitals, said Amy Louviere, a spokeswoman for the mine safety agency. Nonetheless all have been released.

Star Mine Operations of Denver was the owner of the mine; the mine has been cited for many federal safety violations from the time when the company began operating it in 2011. A lot of of the violations involved the misuse of electrical equipment and machinery, or a failure to follow safety precautions, federal mine safety records show. On Oct. 22, which was the latest incident, federal inspectors cited the company for failing to secure gas cylinders safely and for using defective equipment.

According to the mine safety agency, the rate of workdays lost to nonfatal accidents at the mine was more than double the national average for each of the past two years. Rory Williams, the mine’s manager (not related to Rick Williams) did not return phone messages right after but according to The Watch Newspaper in Telluride, the defended the mine’s safety record. The miners who died were equipped with self-rescue and oxygen devices, Mr. Williams said at a news conference Sunday night. Equipment malfunction was not suspected as having played a role, he added. As of this year the two deaths brought the number of mining fatalities in the United States to 36.

Investigators were trying to gather into whether the mine was sufficiently ventilated.

Ellen Smith, the owner and managing editor of Mine Safety and Health News, which tracks mining accidents, said there did not appear to have been any warning signs of potential gas leaks. There were other concerns, she said. On four occasions this year, complaints about hazards were lodged, according to federal records.

Those complaints resulted in a single citation. But Ms. Smith said the fact that miners had repeatedly raised concerns was unusual, especially at a relatively new operation. Gas leaks are not rare in underground mining; however they do not usually cause as mugh deaths as blasts or cave-ins. Bruce Dial, a former federal mine inspector, said it was possible that the minors had encountered carbon monoxide that had lingered for years from previous mining operations. Another possibility, Mr. Dial said, was that carbon monoxide released by the mine’s own operations may have built up.