MEXICO CITY — A deadly riot broke out during family visiting hours Thursday at a prison in the violence-plagued state of Sinaloa in northwestern Mexico, state authorities said.

The violence occurred Thursday morning at the El Castillo Penitentiary in the port city of Mazatlan.

Sinaloa’s Public Security Secretariat said on X that nine inmates and one family member were killed in the incident, while 16 prisoners were taken for medical treatment after being injured. Local media reported that inmates used firearms during the fight.

Authorities have not yet said how the deaths occurred.

Following the clash, members of the army, National Guard, navy and state security forces entered the prison to restore order. Mexico’s federal security cabinet said the prison, which houses 1,800 inmates, is now “under control.”

Overcrowding, driven in part by the use of pretrial detention — which keeps thousands of prisoners behind bars without a sentence — combined with insufficient prison staff, has fueled corruption and violence inside prisons, according to analysts. Last May, a riot broke out at a prison in Culiacan, the capital of Sinaloa, leaving seven inmates dead and another injured.

Mexico has 276 prisons, including 14 federal penitentiaries and 249 state prisons. The prison population reached 261,388 people as of February, according to official figures.

Violence erupted in Sinaloa in September 2024 amid an internal power struggle between two factions of the Sinaloa Cartel, pitting the sons of convicted former drug lord Joaquín “El Chapo” Guzmán, known as “Los Chapitos,” against followers of another detained former cartel leader, Ismael “El Mayo” Zambada.

The Thursday riot comes as Mexico’s government faces pressure from the Trump administration to crack down on cartels.