At least 44 people died in a fight between rival gangs at a prison on the outskirts of the northern Mexican manufacturing city of Monterrey on Sunday, authorities said.
Victims were beaten, stabbed and stoned when a fight broke out early on Sunday, said Jorge Domene, security spokesman for the state government.
Inmates at the prison in Monterrey, about 140 miles (225 km) from the border with Texas, include members of Mexico's Gulf cartel as well as the feared Zetas cartel. It is not clear if the fight was between those two groups, Domene said.
We hope that once the bodies are identified, we'll be able to say who was responsible for the attack, he added.
The prison was secured at about 6 a.m. (1200 GMT) and an investigation began shortly afterward, Domene said.
Prisons in the region are plagued by overcrowding, corruption and mass escapes.
A fire in a prison in Honduras last week killed more than 350 inmates.
In Mexico, where prisoners held on federal drug charges are mixed with common criminals, the prisons are also troubled by violence tied to the powerful drug cartels battling for control of smuggling routes along the U.S.-Mexican border.
During another fight between rival gangs in January in a prison in the northern part of the country, 31 inmates died and 13 were wounded.
About 50,000 people have been killed in Mexico in the past five years since President Felipe Calderon launched an offensive against drug gangs.
Monterrey, the capital of Nuevo Leon state, is a wealthy city that has been plagued by violence as Calderon escalated the war on drug trafficking.
(Reporting by Adriana; Barrera and Elinor Comlay; Editing by Peter Cooney and Stacey Joyce)