Mexican police found the heads of three men Thursday on a traffic circle in Michoacan, a western state where the government deployed troops in May to contain drug cartel violence.
The gruesome discovery was made by municipal police during a morning patrol at the entrance of the town of Los Reyes, said state prosecutor's office spokesman Alejandro Arellano.
Authorities are investigating the motive but all three had bullet wounds, a "coup de grace" suggesting the killings may be a settling of scores, Arellano told AFP.
"The rest of the bodies of the murdered and decapitated subjects were not found. Patrols were launched to find them," the spokesman said.
The victims are believed to be between 28 and 30 years old but their identities remain unknown.
Four people were killed in Los Reyes on July 22 when gunmen shot at at some 300 demonstrators protesting against police corruption.
Residents in several towns of Michoacan, including Los Reyes, formed civilian "self-defense" forces this year to combat violent crimes committed by the Knights Templar cartel.
The vigilantes of Los Reyes disbanded after the government deployed thousands of soldiers in May to control the violence.
Tens of thousands of people have died in drug-related violence in Mexico in the past seven years.
Elsewhere on Thursday, at least two masked gunmen killed four young people and wounded two others outside the northern industrial city of Monterrey, said the Nuevo Leon state prosecutor's office.
Witnesses said that the gunmen arrived in a taxi before killing two people outside a bar and two more inside in the municipality of Santa Catarina.
Monterrey was the scene of a ferocious turf war between the Gulf and Zetas drug cartels in recent years but violence has fallen in the past few months following the deployment of troop reinforcements.