One of the men killed at the Revenue-Virginius mine in Ouray, Colo., on Sunday was trying to find the other miner who died.
New details of the incident from the Mine Safety and Health Administration were released Monday. The agency says in a statement that "preliminary information" indicates "that a miner entered an area of the mine where an explosive had been previously detonated."
A mine foreman noticed that the miner didn't return, the statement says, and succumbed while trying to find the missing worker. Mine rescue teams found the two victims and "detected fatal levels of carbon monoxide gas." The rescuers brought out at least 19 others who were sickened by the gas and taken to hospitals.
MSHA adds that all of the injured have been released and that four agency investigators are "on the ground" trying to determine the source of the lethal gas.
Fires and explosions typically produce carbon monoxide in underground mines, according to MSHA's website.
The investigators won't go underground until the mine is safe, according to MSHA spokeswoman Amy Louviere, who says the agency has "required the mine operator to submit a plan for the rescue teams to re-enter the mine in order to determine that it is properly ventilated and all harmful gases have been removed."
Louviere adds that investigators will very likely be "looking at a variety of things related to ventilation and gas."
Mining companies manipulate ventilation underground so that lethal gases are swept away. Miners are also required to carry gas detectors and emergency breathing masks known as self-rescuers. Investigators will try to determine whether the ventilation system worked properly and whether miners carried working detectors and self-rescuers and knew how to use them.
Mine owner Rory Williams told reporters Monday night that "the fallen miners were equipped with safety equipment, including self rescue devices and rebreathers containing oxygen, and said that equipment malfunction is not suspected at this time," according to The Watch, a local weekly newspaper.
The Watch also reported that Williams said safety is the mine's "number one priority."
"I believe our safety record has been strong," Williams said. "We have not had any incident of this nature ever, and I never intend to have another one."
NPR's review of federal mine safety records confirms that no fatal accidents have occurred at the Revenue-Virginius mine since recent operations began in 2011.
But the most common measures of safety show that the mine is above the national average for underground metal-nonmetal mines in both injuries and safety violations.
The rate of violations (measured in violations per inspection day) during the past 15 months is nearly double the national rate. The injury rate (measured in workdays lost to injuries) was nearly five times the national rate last year and double the average this year.
MSHA records list 25 violations and eight injury accidents since 2011. Twelve of the violations were labeled "significant and substantial," indicating the potential for serious injury or death.
Citations included in the violations noted moderate or high negligence 28 times. Eight citations said the violations were reasonably likely to cause permanent injury or death.
When the mine first reopened in 2011, the company was cited for failing to train workers, and federal inspectors ordered the untrained miners to leave the mine.
But none of the violations appear to cite any problems with the ventilation system or failure to carry, use and have working self-rescuers and gas detectors. There were no additional citations involving proper training after the 2011 incident.
Louviere says MSHA "will conduct a thorough investigation of the accident" but gave no time frame for a report.