MEXICO CITY — Gunmen burst into a cock fight and opened fire, killing 20 and wounding four, prosecutors in Mexico’s western state of Michoacan said Monday.

Three women — including a Chicago area mother — were among the dead after the attack late Sunday near the town of Zinapecuaro.

Melissa Silva and her sister, Arleth Silva, 16, were visiting family in Michoacan and were at the gathering during the shooting. Melissa Silva was killed. Arleth Silva was shot multiple times and is currently in critical condition at a Mexico hospital. 

The attackers apparently planned the attack, and entered in a stolen truck owned by a snack food company, according to prosecutors.

“The snack food company truck arrived, and several armed people in camoflauge clothing got out,” prosecutors said. At the same moment, a bus that was outside the building was used as a blockade, apparently to prevent victims from escaping or calling for help, a statement by prosecutors said.

Investigators found 15 vehicles that apparently belonged to the victims, one of which bore stickers with the logo of a gang.

Prosecutors said drug cartels and other gangs had been fighting in the area.

“There are indications that the attack involved a confrontation between criminal groups,” the federal Public Safety Department said in a statement. It added that a team of federal investigators had been sent to the scene.

Melissa Silva lived in Warrenville and left behind four children. Her family is trying to get her sister back to the states for medical care.

A GoFundMe has been created by family members to help fund Melissa Silva’s funeral and pay for her sister’s health expenses.

Michoacan has been the scene of a longstanding turf battle between local cartels and the Jalisco cartel, from the neighboring state of Jalisco. The fighting has included the use of bomb-dropping drones, landmines and home-made armored cars.

Cock fighting, while illegal in many areas, remains a popular pastime in parts of Mexico, though the fights are usually held clandestinely.