Thirty-one inmates have died in a fight between gang members in a prison in northern Mexico, officials say.
They say the prisoners were killed with knives and makeshift weapons at the prison in the city of Altamira. At least 13 others were injured.
Altamira is in Tamaulipas state, where the Zetas and Gulf cartels have been fighting a bloody war for control of smuggling routes into the US.
Gang rivalries frequently spread into Mexico's prison system.
Many of Mexico's prisons are overcrowded and plagued by violence linked to drugs cartels.
More than 40,000 Mexicans have died in drug-related violence since December 2006, when President Felipe Calderon began deploying troops to combat the cartels.