A fight between rival gangs at a prison on the outskirts of the northern Mexican manufacturing city of Monterrey left at least 44 dead on Sunday, authorities said.
Victims were beaten, knifed and stoned when a fight broke out early on Sunday, the officials said.
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. Authorities could not confirm if the fight was between those two groups.
The prison was secured about 6 a.m. (1200 GMT) and an investigation began shortly afterward, said Jorge Domene, security spokesman for the state government.
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 plagued by violence tied to the powerful drug cartels battling for control of smuggling routes along the U.S.-Mexico border.
A fight between rival gangs in another prison in the north of the country left 31 inmates dead in January and 13 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.
(Reporting by Adriana; Barrera and Elinor Comlay; Editing by Peter Cooney)