A task force has been set up to investigate a scandal in which the death toll from a gas pipeline leak had been covered up at a north China steel plant.

Thirty workers were poisoned while assembling a furnace at 11:45 a.m. on Monday at the Puyang Iron and Steel Co., Ltd. in Yangyi Township of Wu'an City, in Hebei Province, when the accident happened.

The company originally reported on Tuesday that 16 workers were poisoned in the accident, of whom seven died and the other nine were injured.

The death toll was confirmed to be 21 on Friday after company executives admitted covering up 14 deaths.

"The provincial government has set up an investigation team made up of work safety, public security, procuratorate and construction authorities. It is headed by vice provincial governor Sun Ruibin," said Gao Shenxian, director of the Hebei Provincial Administration of Work Safety, late Friday.

"They will investigate the cause of the accident, the actual casualties, whether there are more cover-ups, and other things," he said.

The accident was reported to Wu'an Municipal Administration of Work Safety at 8:20 p.m. Monday. The administration had dispatched officials to hospitals where the injured were being treated to investigate the case.

The police arrested Zhang Fugui, deputy general manager of the steel company, and put Liu Qinglin and Pei Qingfeng, who were in charge of work safety at the plant, under home surveillance.

Wu Zhisen, deputy director of Wu'an Municipal Administration of Work Safety, Wang Xueshu, head of Yangyi Township, and Li Haisheng, deputy head of Yangyi Township, were sacked for failing to ensure work safety and to detect the cover-up.

An initial investigation indicated that installation workers' negligence led to the accident as they had failed to cut off gas supply after they finished welding the pipeline.

Puyang is a private company with more than 8,000 employees, among the top 500 manufacturing companies in China.

The installation workers were from a company based in Nanjing, capital of east China's Jiangsu Province.