Seven people were killed in a gas poisoning case at a paper mill in Guangdong province Friday night, local authorities said Saturday.

Firefighters received the report of the poisoning at Shuangzhou Paper Co., Ltd. in Zhongtang township in the city of Dongguan, the city's publicity department said. Nine workers were then trapped inside a sewage adjusting tank of the paper factory.

Firefighters pulled them out and sent them to hospital. But seven died after medical treatment failed and the other two are in stable condition.

Local government is investigating the cause of the case.