DATONG -- A fire led to Wednesday's gas poisoning in northern Shanxi Province which trapped 45 miners - but the cause of the fire is still unknown, a local government official said on Thursday.

The fire produced mainly carbon monoxide gas in the tunnel, said Zhang Riming, head of the rescue headquarters for the accident, adding that an investigation team is trying to determine the cause of the fire.

Zhang was previously quoted as saying the miners failed to carry out production according to the mining map and they might have accidentally opened up the gas pool.

Forty-five miners were trapped when the incident occurred at the Danangou Coal Mine in the city of Datong at noon on Wednesday. All the miners were rescued but one of them died in hospital, said Zhang.

"Most of the miners have been discharged from hospital, but two peole who suffered serious poisoning are still in danger," he said.

Most of the miners are from the provinces of Sichuan and Shaanxi, according to Zhang.

With an annual production of 90,000 tons, the Danangou Coal Mine is a qualified coal mine and possesses all the required licenses for operation from the local government.

The mine owner, who was also injured in the accident, is under the surveillance of the local public security department.

Further investigations are underway.