Mexican police on Wednesday found four human heads lined up at the feet of a statue in Apatzingan, in the western state of Michoacan.
The attorney general's office said the heads, all of which belonged to men, had been laid at the feet of a statue of former Mexican president Lazaro Cardenas (1934-40) at a roundabout where there is typically a lot of traffic.
Messages, whose content was not made public, were left besides the heads. They were signed by the group La Resistencia, which officials link to the violent, Michoacan-based drug cartel La Familia.
According to media reports, 18 beheaded bodies have been found so far this year in Apatzingan, an area where clashes and vendettas are frequent among drug gangs.
More than 7,700 people were killed last year in Mexico in incidents linked to organised crime, particularly drug trafficking.