At least 23 inmates have been killed after a fight at a jail in Durango, northern Mexico, officials say.
Police believe the inmates were members of rival drugs gangs, Reuters news agency reported.
But prison spokeswoman Carla Puente was quoted by the Associated Press as saying she did not know the cause of the fighting.
Thousands of people have been killed in drug-related violence in Mexico over the past few years.
"There are 23 dead, all inmates," Durango army head General Moises Melo Garcia told Reuters.
He said the situation in the prison was "now back under control".
Ms Puente said some people had also been injured in the brawl, which was ended by prison guards and armed soldiers.
The prison was reported to house more than 2,000 inmates.
The state of Durango has been the scene of clashes between the rival Gulf and Sinaloa drug cartels in recent months.
The drugs trade in the region is controlled by the Sinaloa cartel, reportedly led by Mexico's most-wanted man, Joaquin "El Chapo" Guzman.
At least 19 inmates died in another prison brawl in Durango in August 2009, which officials said had been sparked by gang rivalries.
President Felipe Calderon has declared war on the illegal narcotic organisations in Mexico, deploying 40,000 troops to fight the cartels.