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.