Four people critically injured in a fatal explosion in Dangyang, Hubei province, were transferred to hospitals in the provincial capital, Wuhan, on Friday, according to local authorities.

Twenty-one people, ages 22 to 51, were killed and five people were injured in the incident, which happened at Dangyang Madian Gangue Electricity Supply Co on Thursday afternoon. All of them were company employees, according to the local government public relations department.

The dead employees have been identified and DNA tests have been conducted, the department said.

The company's legal representative has been detained and several other people are being investigated by the local police, it added.

A statement issued by the State Administration of Work Safety said the cause of the blast had yet to be determined,

The blast happened at 3:20 pm on Thursday, when a high-pressure steam pipe ruptured as workers were testing thermo-power facilities, a preliminary investigation showed.

Villager Du Qiuxin, who lives about 1 km from the plant, told Xinhua News Agency that there was a loud bang when the blast occurred.

"I rushed out to check what had happened. The area was quickly sealed off and a lot of police officers and firefighters arrived," she said.

The local authority said the plant became operational earlier this year and its facilities are still undergoing test operations,

One of the critically injured workers, a 32-year-old surnamed Liu, has worked for the plant's co-founder, Dangyang Huaqiang Chemical Co, for nine years, according to Xinhua.

"He began to work at the power plant just a month ago. It's a huge blow to the family. His father died last year and his daughter is only 8-years-old," Liu's uncle said.

All the plant's facilities have been shut down, and work has been suspended at neighboring companies.