Police found 13 bodies piled on top of one another outside a convenience store in western Mexico on Monday, local and federal authorities said.
The corpses had been shot in what appeared to be a flare up in an on-going turf war between drug cartels in Michoacan state.
The victims, who were discovered semi-naked along with threatening messages, were left on a highway on the outskirts of Zitacuaro, a city some 160 kilometers (100 miles) west of Mexico City, the state attorney general's office said.
Local emergency services said the victims, at least three of whom were minors, showed signs of decomposition, suggesting they had been killed days earlier and then dumped outside the store in the early morning hours.
Michoacan, the home state of President Felipe Calderon, has been relatively calm since police killed the boss of the cult-like La Familia cartel in December 2010.
The state had been plagued with violence as rival gangs fought over lucrative methamphetamine trafficking routes and marijuana fields across western Mexico.
More than 46,000 people have died in drug-related violence since Calderon launched an army-led crackdown on the cartels after taking office five years ago.
Zitacuaro is a city used by tourists as a launching point to visit nearby butterfly sanctuaries during the annual monarch migration in October and November. (Reporting by Julie Gordon and Lizbeth Diaz)