The miners who died of carbon monoxide poisoning at the Revenue-Virginius gold, silver and lead mine in southwest Colorado on Sunday, November 17, apparently entered an area where a blast was detonated, according to federal investigators; the 19 others sickened in the incident have since recovered.

Preliminary information indicates that at 7:20 a.m. one of the men entered the area first, and that, after he didn’t return, a foreman went searching for him, according to U.S. Mine Safety and Health Administration (MSHA) officials. Both men were discovered by other miners in the area; they, in turn, were immediately evacuated.

Although one of the miners who inhaled the gas was initially in critical condition, all 19 have since been released.