When the first miner didn't return, she said, a foreman went looking for him. Both men were later found by other miners working in the area, and those miners immediately evacuated the underground gold, silver and lead mine, she said.
Mine manager Rory Williams previously said there was a blast in the mine to remove rock on Saturday that may have been a source of the carbon monoxide. Louviere said the blast occurred during the previous shift, so it likely happened on Saturday.
Williams, who isn't related to Rick Williams, said he knew both miners.
"They were great men. They will be remembered indeed," he said.
Nineteen miners were taken to a hospital for treatment and all have been released.
About 100 miners work at the site, about 270 miles southwest of Denver, which will be closed until the end of the investigation.
At a news conference late Sunday, Rory Williams said all of the miners had air packs to help them breathe in an emergency, but he didn't know whether they carried gas detectors to alert them to dangerous levels of carbon monoxide. He said there didn't appear to be any equipment malfunction.
He didn't return a call Monday seeking further details.
Regulators weren't aware of any previous carbon monoxide problems.
Carbon monoxide exposure is a longstanding problem within the mining industry the invisible, odorless "silent killer" can quickly overwhelm its victims at high concentrations, said Tom Hethmon, a mine safety expert from the University of Utah. It results from the incomplete combustion of gas, wood or other fuels, and in mines can come from machinery or the use of explosives to blast rock.
Improvements in ventilation practices have helped reduce accidents, Hethmon said. But if ventilation systems fail or the volume of gas is too great, carbon monoxide can overwhelm those exposed in a matter of seconds, he said.
Short-term symptoms of exposure include dizziness, confusion and headache. Miners are typically required to carry or have ready access to portable, self-rescue breathing devices that provide enough oxygen to last about 45 minutes to an hour.
Robert Ferriter, a senior mine safety specialist at the Colorado School of Mines, said routine air tests done after blasting should have detected any problems from earlier blasting at the Revenue mine.
"The big question here is how you got 20 people exposed to such a massive amount of carbon monoxide," he said.
The mine dates to the 1870s and was taken over by its current owner, Denver-based Star Mine Operations, in 2011. The company, which also has a mine in Cerro Blanco, Peru, received a permit to resume mining at the site near Ouray in February 2013. All the mining is done by drilling and blasting, according to state regulators.
Federal records show the mine reported eight employee accidents in the past two years, including five that resulted in days away from work for the injured worker.
The mine has received 33 citations since Star Mine became the operator, including 13 that regulators determined were reasonably likely to result in injury or illness. The most recent came last month after inspectors found tire chains stacked in a walking area that created a tripping hazard.