Nineteen inmates died and three were injured on Wednesday in a prison riot in the Mexican border city of Ciudad Juarez, the police said.
The trouble first started with rival gangs of inmates fighting for control of the prison. Anti-riot police were brought in as the situation spiralled out of control.
There was additional monitoring support from military helicopters that circled the jail, which is about 30 km south of Ciudad Juarez.
The fighting was sparked by Ciudad Juarez Mayor Jose Reyes' decision to transfer control of the jail to the military.
Ciudad Juarez, which is just across the border from the US city of El Paso in Texas, is among the most crime-ridden cities in Mexico, rife with gangs involved in drug trafficking, extortion and kidnapping.
Last week, Mexican President Felipe Calderon ordered the deployment of an extra 5,000 military troops to assist the 2,000 troops already in the city.