Two workers have been killed and 20 injured in a mining accident in Colorado.
The miners were trapped underground at the Revenue Virginius mine near the south-western Colorado town of Ouray.
Star Mine Operations, the owner of the mine, could not be reached for comment, but Ouray County sheriff's office said the company has accounted for all workers at the site.
Twenty people were taken to local hospitals and all but two have been treated and released.
The circumstances of the accident and the conditions of the workers treated in hospital have not yet been released.
Rory Williams, project manager for Star Mine Operations, told the Ouray Watch the accident appeared to be a "powder-smoke incident", and that the release of chemicals in the smoke injured the miners.
The US Mine Safety and Health Administration is at the accident site.
Authorities said later that the two miners who were killed died of carbon monoxide poisoning.
The Denver Post reports that officials said the source of the gas was under investigation. They are looking at whether a small explosion in the mining process might have caused it.
The Montrose Daily Press reports that 10 people went to Montrose Memorial Hospital, where officials say they were treated for carbon monoxide exposure.
Rory Williams, the operations manager for Denver-based Star Mine, said all of the men are required to wear personal respirators and the two who died had them. He said it does not appear to be an equipment malfunction.