MONTERREY, Mexico: Relatives of 49 inmates slaughtered in a fiery Mexican prison brawl sobbed and fainted as they read the list of victims on a wall after the country’s deadliest jailhouse riot in years.

Thursday’s battle involved two rival factions of the Zetas drug cartel fighting for control of the prison in the northern industrial city of Monterrey, highlighting the lack of authority and overcrowding plaguing many Mexican prisons.

Hundreds of relatives flocked to Topo Chico prison before dawn after they learned that a brutal battle and fire had broken out.

They shouted desperately through a fence to seek information from inmates while other relatives briefly pulled open the main gate and tossed rocks in anger.

But then an official placed the grim list of victims on a wall outside. Some bawled and fainted while one woman was carried out in an ambulance.

“All this for a robbery, my son was innocent,” a woman cried after seeing her son’s name on the list of the dead.

Twelve others were injured during the “pitched battle” that lasted 30 to 40 minutes, said Nuevo Leon state Governor Jaime Rodriguez.

“They used sharp weapons, bats, sticks,” the governor told radio Imagen, adding that the 60-year-old penitentiary houses 3,800 inmates, overseen by 100 guards. The National Human Rights Commission says the prison has room for 3,635 convicts.

One of the inmates was shot dead by a prison guard who was protecting a group of women, Rodriguez said.

During the violence inmates set a fire in a supply room and TV images showed flames coming out of the prison in the middle of the night.

Milenio TV showed video made by people living near Topo Chico of one inmate falling to the ground as others beat him with sticks in the prison yard.

The riot erupted on the eve of Pope Francis’s five-day trip to Mexico, during which he is due to visit another notorious prison in the northern border city of Ciudad Juarez.

But National Security Commissioner Renato Sales said the pope would visit a safe prison, telling Radio Formula that the facility is a “national exception” because the Chihuahua state authorities have established an “orderly and disciplined” penitentiary system.