MONTERREY, Mexico — A brawl between rival drug gangs at an overcrowded penitentiary in northern Mexico turned into a riot Thursday, leaving 49 inmates dead and 12 injured in the country’s deadliest prison melee in years.

No escapes were reported in the clash at the Topo Chico prison in Monterrey, said Nuevo Leo state Gov. Jaime Rodriguez. The riot took place on the eve of Pope Francis’ arrival in Mexico, a visit that is scheduled to include a trip next week to another prison in the border city of Ciudad Juarez.

At a news conference the governor read a list of 40 names of confirmed victims, saying five of the remaining bodies had been charred by fire and four were yet to be positively identified. One of the injured was in grave condition.

Terrified relatives gathered at the prison gates, where officials posted the names of the dead as they became known.

The fighting began around midnight with prisoners setting fire to a storage area, sending flames and smoke billowing into the sky. Rescue workers were seen carrying injured inmates – some with burns – from the facility.

Rodriguez said the clash was between two factions led by a member of the infamous Zetas drug cartel, Juan Pedro Zaldivar Farias, also known as “Z-27,” and Jorge Ivan Hernandez Cantu, who has been identified by Mexican media as a Gulf cartel figure.