MEXICO CITY – A group of assailants shot and killed 15 people, including a farm workers’ organizer and three minors, at a ranch in the northwestern state of Sonora, a local official said.

The results of the initial investigation indicate the gunmen were targeting Margarito Montes Parra, the leader of the Ugocep rural workers’ union, who had been away from Sonora for five years due to previous threats and had only returned to the state a month and a half ago.

One of Montes Parra’s sons had also been killed in Sonora five years ago.

The farm leader, his wife, their young son, friends and family members were traveling Friday in two pickup trucks to “El Alamito” ranch outside the town of Cajeme for a family celebration, said Antonio Jose Rodríguez, spokesman for the local mayor’s office.

The group of assailants, armed with assault rifles and traveling in four SUVs, were waiting for Montes Parra’s arrival and fired almost 1,000 rounds at him and the other members of his group, based on evidence found at the crime scene.

Only one person survived Friday’s ambush at the ranch, which was owned by the farm leader.

A source close to the investigation said that among the victims was the wife of the Ugocep leader, Gabriela de Montes, and the couple’s 15-year-old son.

Municipal, state and federal police have been dispatched to the ranch, as well as army soldiers who are investigating the killings. Authorities have not yet determined who was behind the massacre, although the type of weapons used and the modus operandi suggest it was the work of an organized criminal outfit.

For his part, Jose Luis Gonzalez Aguilera, the national leader of the Permanent Agrarian Congress, a coalition of the largest farm workers’ organizations; and Max Agustin Correa, leader of the Cardenist Peasant Central, called on the Sonora state government to work swiftly to clear up the murders.

Sonora – which borders the United States, the world’s biggest market for illicit drugs – has emerged as a new battleground in the cartels’ battles over smuggling routes. It is being fought over by the Beltran Leyva brothers’ cartel and the Sinaloa drug mob, led by Joaquin “El Chapo” (Shorty) Guzman.

Sonora was the scene of one of the bloodiest episodes in Mexico’s drug war in 2007, when a group of some 40 armed men moved into the town of Cananea and became engaged in a gun battle with police that left 23 dead, among them 16 drug cartel enforcers.

Violence related to organized crime has claimed more than 15,000 lives in Mexico since December 2006, when newly inaugurated President Felipe Calderon deployed tens of thousands of troops and federal police to battle the country’s powerful drug cartels.