YINCHUAN - The bodies of four workers were pulled from a well Saturday after the men were allegedly poisoned to death by a gas leak while working at a landfill in northwest China's Ningxia Hui autonomous region.

Around Saturday noon Zhang Lei, director of the Dawukou Landfill in the city of Shizuishan , carried out a routine check in an observation well. He reportedly discovered that something was wrong with a gas valve and called another worker to enter the well to repair it, said a statement of the city government issued on Sunday.

As the two did not reappear from the well, two other workers descended into the well around 4:30 p.m. Local fighters later entered the well and retrieved dead bodies of the four men.

A preliminary investigation indicated the landfill workers were poisoned by toxic gas due to improper working operations and wrong rescue measures, said the statement.

But it said the exact cause of the accident was still under investigation.