Nearly a dozen people were killed Thursday after a prison gang fight broke out that quickly spread to the streets of Ciudad Juarez, just across the U.S.-Mexican border from El Paso.

Two inmates were killed in a fight before the prison riot spread into the streets, where alleged gang members killed nine more people, including four radio station employees, the Associated Press reported.

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

Authorities in the border state of Chihuahua have called in the army and the National Guard to control the fight, according to the prosecutor’s office. The office explained the conflict at No. 3 prison was between “rival gangs,” but it did not specify which gangs were involved. Prosecutors initially reported three dead in the fight but later changed their report to two and four inmates injured.

Years of conflict have raged in Ciudad Juárez between gangs such as the Artistas Asesinos, backed by the Sinaloa cartel, and the La Linea and Aztecas gangs, as well as the Juarez cartel.