QUITO, Ecuador (AFP) — Authorities said at least 27 inmates were found dead by asphyxiation Sunday afternoon at an Ecuadoran prison where an armed riot left four others dead and dozens injured in the morning.

In a statement, prison authorities said the 27 individuals found dead at the Machala prison in El Oro province had “among themselves committed asphyxiation, which caused immediate death by suspension.”

Authorities said they were still working to “fully clarify the facts,” and forensic medical personnel were on site to verify information.

The deadly day at Machala’s prison marks the latest spasm of prison unrest in the South American country.

Ecuadoran 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 morning incident broke out around 3 a.m. at the prison in Machala, a southwestern city.

Local residents could hear 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 morning violence, while 33 inmates and one police officer were injured.

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.