The leader of a Mexican farmworkers' organization and 14 other people were killed in a mass shooting in the northern Mexico state of Sonora, an official said Saturday.

Sonora prosecutors' spokesman Jose Larrinaga said the victims include farm leader Margarito Montes, 10 other men, one woman and three minors. Most were believed to be Montes' relatives or employees.

Larrinaga said the victims' bullet-ridden bodies were found on a roadside near a farm Friday. The killers apparently used assault rifles, the sort of weapon favored by Mexico's drug gangs, but Larrinaga said the motive in the attack was still under investigation.

The killings occurred in a rural area of the state, near the border with Sinaloa, the home state of some the country's most powerful drug lords.

Montes was the leader of the General Popular Union of Workers and Farmers, which claims tens of thousands of members, mostly in southern Mexico. Local news media reported that Montes had led peasant and squatters' movements involved in land seizures and that his group has sometimes had violent clashes with rival claimants to land.

Montes' group is an affiliate of the nationwide Permanent Agrarian Congress, which said Montes was ambushed by a gang of gunmen near his home.

It issued a statement calling his killing "a cowardly assassination," and suggested it could have been related to land disputes.

The group said it "regrets .... that in the 21st-century, land disputes are still being settled with gunfire" and demanded authorities catch and punish the killers.

In the southern state of Guerrero on Saturday, state police reported assailants tossed two grenades at a police guard station at a toll booth in the resort of Acapulco.

Assailants also fired assault rifles and tossed two grenades at a police station in the nearby town of Tecpan de Galeana. Some damage but no injuries were reported in the attacks.