Mexico City (CNN) -- Eleven inmates died in a fight at a Mexican prison Tuesday afternoon, the public safety ministry in Durango state said.
The dead inmates appeared to have stab wounds, but prosecutors had not determined an official cause of death, a ministry statement said.
The fight occurred at the same prison where last year the Mexican attorney general's office alleged that inmates were let out to commit killings of rival drug gang members.
An alarm in the prison in Gomez Palacio, Durango, sounded around 4:45 p.m., the safety ministry said, after a brawl began in a processing area.
State prosecutors were investigating what caused the fight.
In July, top officials in Mexico said that authorities at the prison released and armed several inmates to attack a group of people during a birthday celebration in a shooting that left 17 dead.
Ricardo Najera, a spokesman for the attorney general's office, said authorities allowed a group of inmates to leave the Cereso prison in police vehicles to launch the attack.
Four top prison workers -- including its director -- were named as suspects in the investigation, Najera said.
Police traced the weapons used in the July 18 incident to other violent attacks, Najera said.
CNN's Catherine E. Shoichet and Ariel Crespo contributed to this report.