A Datong City official said investigators suspect spontaneous ignition or a gas blast likely caused the carbon monoxide to be released into the Wujiu coal mine, which is now the target of a search for survivors, Xinhua, China (OTCBB:XHUA)'s state-run news agency, reported Sunday.

Four miners were helped from the coal mine following Saturday's accident and another seven miners escaped on their own, the official said.

The unidentified official said it was still unknown exactly how many miners had been working underground when the deadly incident occurred.

Xinhua said the Wujiu mine has an annual production capacity of 150,000 tons of coal and is in the heart of Datong, China's so-called "capital of coal."