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A gang riot inside a border prison that left two inmates dead quickly spread to the streets of Ciudad Juarez where alleged gang members killed nine more people, including four employees of a radio station, security officials said Friday.

The federal government’s security undersecretary, Ricardo Mejía Berdeja, said the violence started inside the state prison after 1 p.m. Thursday, when member of the Mexicles gang attacked members of the rival Chapos.

Two inmates were killed and 20 injured.

Then suspected gang members outside the prison began burning businesses and shooting up Ciudad Juarez.

“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.

The violence came two days after drug cartel gunmen burned vehicles and businesses in the western states of Jalisco and Guanajuato in response to the arrest of a high-ranking cartel leader.