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The explosion occurred in an expressway tunnel under construction in Hunan province

Four workers were pulled from the debris, state media said

Beijing CNN  — 

At least 20 people were killed Saturday in an explosion inside an expressway tunnel in central China’s Hunan province, state media said.

The tunnel in Zhuzhou city was still under construction with 24 workers on site. Four of the workers were pulled from the debris, one in critical condition, said a statement from the Hunan provincial transport department that was cited by the state-run Xinhua news agency.

The blast occurred at 8:30 in the morning when a vehicle was unloading explosives needed for construction purposes.

An investigation was under way, Xinhua said.