At least 44 people have died in a prison fight in northern Mexico.
Security officials said members of rival drug cartels confronted each other with stones and home-made weapons in the jail north of the city of Monterrey.
Some victims were strangled.
An investigation is under way to establish if some of the prison guards colluded in the fight by unlocking the doors separating two wings of the prison.
Deadly fights between rival gangs are not uncommon in Mexico's notoriously overcrowded and corrupt prison system.
Nuevo Leon state security spokesman Jorge Domene said the fight was sparked by the bitter rivalry between the Gulf cartel and the Zetas drug gang.
The two groups are fighting a brutal war for control of the drug smuggling routes to the United States.
That violent rivalry is mirrored in the region's prisons, where rival gang members live cheek-by-jowl.
"We've confirmed 44 dead, all from the prison's "D" wing, Prison Director Gilberto Cesena said.
The number of those injured in the fight, which started at 02:00 local time, has not yet been given.
Mr Cesena said the security forces were now in control of the situation.
Relatives of the prisoners have gathered outside the jail to find out news about who has been killed and injured.
Mr Domene asked them to be patient.
"We have more than 3,000 inmates in this penitentiary and we have to account for all of them first," he said.
The prison was built to hold around 1,500 inmates. Mexican jails are notorious for overcrowding, corruption and rioting.
In January, 31 inmates were killed at a prison in Altamira, in Tamaulipas state, when rival gangs confronted each other with homemade weapons and knives.