WUHAN - Four workers were suffocated to death Monday while cleaning a waste pulp sink at a paper mill in Central China's Hubei province, local authorities said.

A preliminary investigation indicated they were poisoned by toxic gas, the city government of Xiantao said in a press release Tuesday.

It said the accident happened at 9 am at Shengda Paper Industrial Co Ltd. in Zhanggou Township.

Qiao Bencai, a temporary employee, was sent down the 2.8-meter deep sink at 8:30 am, but passed out while cleaning the waste pulp, the document said.

Three colleagues, including a workshop manager, went down to his rescue, but fainted in the sink, too.

All four had died when they were pulled out.

It said the exact cause of their deaths was still under investigation.

Many paper mills use chlorine gas to bleach pulp for white paper and dangerous pollution is often created as a byproduct.