A prison confrontation between members of two rival cartels spilled on to the streets of the border city Ciudad Juárez, where alleged gang members have killed nine more people, including four employees of a radio station.

The violence began on Thursday, when Los Chapos, members of the infamous Sinaloa cartel formerly led by Joaquín “El Chapo” Guzmán, clashed with the local group Los Mexicles, in a Juárez prison, the deputy security minister, Ricardo Mejía, said.

A riot then broke out, leaving two shot to death and four injured with bullet wounds, Mejía said, speaking alongside the Mexican president, Andrés Manuel López Obrador, at a regular news conference. Another 16 were injured in the fighting, he said.

Officials did not say what caused the clash.

After the riot, the Mexicles rampaged in the city, authorities said, killing nine civilians. Among them were four employees of a radio station, including one announcer, Mejía said.

“They attacked the civilian, innocent population like a sort of revenge,” said 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. 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.

Across town, convenience stores were also shot at and set on fire. FEMSA, the parent company of the Oxxo chain, said in a statement that one of its employees and a woman who was applying for a job were killed in the violence.

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.

About 1am on Friday morning, six alleged members of Mexicles were arrested by local police, with help from the army and national guard, Mejía said.

Ciudad Juárez has long had a reputation for violence. Gangs like those involved in the riot often serve as proxies and street-level enforcers for Mexico’s powerful drug cartels who aggressively exert control over the border crossing routes they need to move their product to the United States.

While still high, murders in recent years were well below what they were more than decade ago – about 1,400 last year compared to more than 3,600 in 2010 – according to data from Molly Molloy, a retired border specialist at the New Mexico State University Library, who has tracked the city’s homicide data for many years and posts regular updates to her Frontera List.

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.

López Obrador, popularly known as Amlo, came to power promising “hugs not bullets”, which he promised would reduce soaring murder rates in the country. But experts say the strategy has only emboldened criminal groups to become more violent and expand territorial control across the country.