Photo taken by cellphone on Aug 11, 2016 shows the explosion site at a power plant in Dangyang, central China's Hubei province. [Photo/Xinhua]

YICHANG - A preliminary investigation has shown that the explosion which killed 21 on Thursday in central China's Hubei Province was caused by a broken pipe, local authorities said.

A high-pressure steam pipe exploded at Madian Gangue Power Generation Co. Ltd. in Dangyang City around 3:20 p.m. At least 21 people were killed and five others injured. Three of the injured are in a critical condition.

The pipe broke during testing, according to the Dangyang city committee of the Communist Party of China.

The investigation is continuing.

Xinhua reporters saw a dozen emergency vehicles parked at the factory gate. A road leading to the area has been blocked off, some ambulances are still arriving at the plant.

The house of villager Du Qiuxin is some one kilometer away from the plant. "I was taking a nap, and was awaken by a loud bang. The sound lasted several minutes," she said.

A nurse with the ICU of Dangyang People's Hospital said that several injured people are in the ICU, all suffering from large-area burns, some with internal injuries.