Mexican authorities have detained 20 inmates for alleged involvement in a prison riot that killed 31 in a state bordering Texas and are investigating whether prison staff were also responsible.
No prison officials had been dismissed so far over the fight in which one cell block invaded another Wednesday afternoon, causing a brawl among prisoners armed with makeshift knives, clubs and stones, said Morelos Canseco, interior secretary of Tamaulipas state.
The fight at a penitentiary in the Gulf coast city of Altamira also injured 13 people.
Canseco told reporters Thursday the bloodshed had nothing to do with prison overcrowding. He said the facility, with a capacity of 1,626 prisoners, housed 1,058 inmates at the time of the brawl.
He said state police moved in to control the riot.
The area near the border with the state of Veracruz has seen a spike in drug violence in the last two months from warring cartels.
In late December, five gunmen attacked three buses in Veracruz, killing seven passengers, including three U.S. citizens, and four others in separate attacks. Twenty-three bodies were found along the border between the two states in the following days.
A U.S. government official in Mexico said the Gulf Cartel is battling its former allies, the Zetas, for control of territory and smuggling around Altamira, a very active port.
The official, who couldn't be identified for security reasons, said he had also heard reports that the prison riot was caused by infighting within the Gulf Cartel, which has been weakened by significant arrests in the U.S., Mexican government pressure and internal disputes.
"We don't have any indication that these incidents are tied together," the official said of the recent cartel violence. "It's very unclear what's happening there ... it's difficult to say on any given day who's in charge of a particular stretch of territory, but it's a war."
Deadly fights happen periodically in Mexican prisons as gangs and drug cartels stage jail breaks and battle for control of penitentiaries, often with the involvement of officials.
Another bloody fight in a prison in the Tamaulipas border city of Matamoros in October killed 20 inmates and injured 12.
In July, a riot at a prison in the border city of Juarez killed 17 inmates. Mexican authorities detained the director and four guards over that fight, where a surveillance video showed two inmates opening doors to let armed prisoners into a room where the slain victims were reportedly holding a party.
In one of the worst cases of corruption, guards and officials at a prison in Gomez Palacio, in the northern Mexican state of Durango, allegedly let inmates out in 2010, lent them guns and sent them off in official vehicles to carry out drug-related killings, including the massacre of 17 people.
A riot at the same facility in 2009 killed 19 people. Twenty-three people were killed in a separate prison riot in Durango city in 2010.
Some 29 inmates were killed in two incidents on the same day in 2010 in Mazatlan in western Sinaloa state, home of the powerful cartel of the same name.
Another Tamaulipas prison riot killed 21 in Reynosa in 2008, while two prison riots in Tijuana that year killed a total of 23.
Canseco said Tamaulipas has been trying to professionalize its prison employees in an effort that predates Wednesday's deadly clash.