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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.

Rodriguez said in the morning that 52 people had died, but he lowered that by three in the late afternoon. The reason for the changed death toll was not clear.

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.

"Ayyy, my son is on the list!" 63-year-old Maria Guadalupe Ramirez screamed when she saw the name of her son, Jose Guadalupe Ramirez Quintero, 26, before collapsing into the arms of a daughter and human-rights workers.

Ramirez's grief echoed the concerns of others whose loved ones were tossed into Topo Chico along with some of Mexico's most hardened criminals, despite being sentenced for minor offenses or even while they were still on trial.

"He had already gotten out. They picked him up again just for drinking. ... There is injustice in this prison," she said, shaking her fists and sobbing.

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.

According to a 2014 report by the National Human Rights Commission, Topo Chico was designed to house 3,635 prisoners but actually held about 4,585 that year.

Leslie Solis, a security and justice researcher at the public-policy think tank Mexico Evalua, said the commission's most recent rating of Topo Chico indicated that "we had it coming" and "all the conditions were in place for this" — too few guards, poor training and the entry of illicit objects and substances.