At least four people were killed and over 30 were injured in violence on Sunday at a prison in Ecuador involving firearms and explosives, authorities said, the latest spasm of prison unrest in the South American country.

Ecuadorian prisons have become operational centers for rival drug-trafficking gangs, with over 500 inmates killed in fighting between groups competing to control the lucrative but illegal trade.

The latest incident broke out around 3:00 am (0800 GMT) at the prison in Machala, a southwestern city. Local residents could hear sounds of gunfire, explosions, and cries for help coming from inside the prison’s walls.

Ecuador’s SNAI prison authority issued a statement saying that four people had died in the violence, while 33 inmates and one police officer were injured.

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Elite police teams entered the prison immediately and regained control after the riot broke out, the agency said.

But it did not specify the identities of the deceased or confirm whether the violence was another case of inter-gang fighting.

Violence is believed to be connected to plans to move some inmates into a new maximum-security prison built by the government of President Daniel Noboa in another province that is due to be inaugurated this month.

At the end of September, another armed confrontation at the prison in Machala left 13 inmates and a prison official dead.