Carbon monoxide has been blamed for the Tuesday deaths of 12 workers in a coal mine in northeast China's Heilongjiang Province, a local official said Thursday.

Three workers entered the Yongsheng mine in Dongning County to pump out water before passing out from carbon monoxide poisoning around 10:30 am Tuesday, said Zhang Fuguang, deputy head of the county government.

Zhang said an initial investigation showed that a dense buildup of carbon monoxide in the mine was caused by self-igniting coal in an old mine located adjacent to the Yongsheng mine.

The mine's managers organized volunteer rescue efforts to locate the workers, but the rescuers were also poisoned, as they had no equipment to detect the carbon monoxide.

More than 30 professional rescuers arrived Tuesday afternoon and pulled 20 people out of the mine, including the trapped workers. Twelve of the people, including the head of the mine, died despite medical treatment.

The other eight are being treated at local hospitals, where they are in stable condition.