Mexican border cities of Tijuana and Ciudad Juarez saw a local gang war turn into a deadly riot this week. At least 11 people have been killed in the incident, which has triggered a massive police response near the US-Mexico border.

Two rival gangs, believed to be Los Chapos and Los Mexicles, started a riot inside the state prison around 1 PM on Thursday. According to Ricardo Mejía Berdeja, the federal government’s security undersecretary, two inmates were killed and 20 were injured in the fight.

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The violence quickly spread through the streets of Ciudad Juarez and Tijuana, killing nine more people, including four employees of a radio station, security officials said Friday.

“They attacked the civilian, innocent population like a sort of revenge,” President Andrés Manuel López Obrador said. “It wasn’t just the clash between two groups, but it got to the point in which they began to shoot civilians, innocent people. That is the most unfortunate thing in this affair.”

Mejía said four employees of MegaRadio who were broadcasting a live promotional event outside a business were killed in the shooting.

Chihuahua state Attorney General Roberto Fierro Duarte said that a boy wounded in a shooting at a convenience store died later at the hospital, two women were killed in a fire at another gas station convenience store and two other men were shot elsewhere in the city. Fierro said 10 suspects had been arrested.

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The United States Consulate in Tijuana issued an advisory for American citizens on Friday, urging them to “shelter in place.”

“The U.S. Consulate General Tijuana is aware of reports of multiple vehicle fires, roadblocks, and heavy police activity in Tijuana, Mexicali, Rosarito, Ensenada, and Tecate. U.S. government employees have been instructed to shelter in place until further notice”, a statement from the Consulate read.