The accident occurred at a mine in Ouray County, Colorado.Nathan Bilow/AP
Two miners were killed and 20 injured Sunday in a carbon monoxide accident at a silver mine in southwestern Colorado, authorities said.
The cause of the accident at the historic Revenue-Virginius Mine in Ouray County, about 330 miles southwest of Denver, was under investigation, according to county spokeswoman Marti Whitmore.
She said authorities had so far ruled out an explosion or cave-in.
The mine is owned by Denver-based Star Mine Operations, LLC, but company officials could not be reached for immediate comment.
Of the 20 miners injured in the accident, Whitmore said one remained in critical condition late on Sunday at a hospital.
The Revenue-Virginius mine was first staked in the 19th century and was re-opened in 2013 after years of inactivity, according the Ouray Historical Society.
It is located at an elevation of 12,000 feet along the so-called Governor Basin in the rugged San Juan Mountains.
There have been eight mining deaths in the state since 2002, not including the two Sunday, according to the mine safety agency.
The last large-scale mining disaster in Colorado occurred on April 15, 1981, when an explosion killed 15 people at the Mid-Continent Dutch Creek No. 1 Mine near Redstone.
In its heyday, between 1876 and the late 1940s, the Revenue-Virginius mine produced more than 14.5 million ounces of silver, according to local newspaper Ouray Watch.
Star Mine Operations acquired the property in late 2011.