A fierce battle between rival drug gangs at a prison in the Mexican border city of Ciudad Juarez on Wednesday left at least 20 inmates dead and three critically wounded, authorities said.
It took guards, police and military reinforcements nearly three hours to contain the unrest. Black smoke drifted from the cinder-block prison and helicopters patrolled overhead as anxious families waited outside for news.
Most of the victims had been beaten or stabbed to death.
The violence comes as the ravaged city is being placed under military control. Ciudad Juarez, just across the border from El Paso, has been the epicenter of Mexico's raging drug war. More people have been killed there than anywhere else -- 1,600 last year and 250 just last month.
In recent weeks, the city's police chief was driven from office by powerful drug traffickers who began slaying cops to underline their threat. The state governor was ambushed; he survived but a bodyguard was killed.
President Felipe Calderon ordered the deployment of additional troops and police officers to Ciudad Juarez, nearly tripling the number, in an attempt to restore law and order.
On Tuesday, city officials announced that the military would also take command of the local, corruption-ridden police force as well as transportation and city prisons.
Wednesday's prison uprising took place at a state facility that was not slated to go under military authority.
Warden Cesar Martinez Acosta stressed that the fighting did not target guards or security forces, and was not part of a breakout attempt. He denied reports that two federal agents were among the dead.
The brawl, which erupted around 6 a.m. at the end of overnight conjugal visits, pitted the Aztecas gang against the Mexicles. Also fighting was the Mexicles' ally, Artistas Asesinos (Murdering Artists).
The Aztecas work with the so-called Juarez cartel, the trafficking syndicate that has controlled Ciudad Juarez for years. It is now locked in deadly competition for the area with the Sinaloa cartel.
All of the dead were from the Mexicles and Artistas Asesinos gangs, authorities said, suggesting that the Aztecas started the fight. One report said members of the Aztecas stole a guard's keys and were able to free their cohorts and begin a rampage into parts of the prison controlled by their rivals.
Police with tear gas backed by the army eventually put down the riot.
At one point, inmates could be seen on the roof of the prison, torching mattresses. Witnesses said they saw bodies being hurled from second-story windows.
Carlos Gonzalez, spokesman for the state public security ministry, said the exact causes of the brawl were not yet known, but that bad blood between the Aztecas and Mexicles runs deep. They fight over control of drugs, guns and other contraband in the prison.
"There have been conflicts between these two gangster groups for a very long time, and there have been fights in the past," he said in an interview. He said guns and sharp instruments were used in Wednesday's melee.
Meanwhile, the military buildup in Ciudad Juarez has continued. An additional 3,200 troops arrived last weekend and more are on the way, officials said Wednesday.
The troops will patrol the city and set up nighttime checkpoints aimed at preventing the free movement of traffickers.
The office of Mayor Jose Reyes Ferriz issued instructions to citizens to approach roadblocks with care, turning on the lights inside their cars and rolling down their windows so that soldiers can identify passengers easily.
Eventually the army will also take up positions in rural areas around Ciudad Juarez to block traffickers from fleeing to neighboring states.