Work is under way on Saturday to rescue five people trapped in a copper mine in northwest China's Gansu Province, according to the provincial work safety bureau.

Officials with the bureau said the five include two water-drainage workers who entered the mine -- owned by Gansu Zhujin Mining Co. Ltd., in Xihe county, Longnan city -- at around 11 a.m. on Friday. They were followed several hours later by another three attempting a rescue.

The site has not been mined since 2009, when it closed, and the company authorities suspect the workers got into trouble when they were poisoned by gas.

That theory should be verified as an investigation has begun into the cause of the accident.