Mexican police found the bodies of 13 men and boys outside a gas station in the central state of Michoacan on Monday, authorities said.
The 10 adults and three minors showed signs of being shot in the head, the Notimex news agency reported, citing state prosecutors.
Police found the bodies stacked outside a gas station convenience store in Zitacuaro, Mexico, around 2:30 a.m., state investigators said in a statement.
Authorities found a written message linked to organized crime at the site, Notimex said.
Forensic investigators are working to identify the victims and confirm the cause of death, regional prosecutors said.
Michoacan is Mexican President Felipe Calderon's home state. Shortly after taking office in December 2006, he announced plans to deploy federal troops there to help the government's fight against organized crime.
Mexican authorities have said cartels operating there include La Familia Michoacana, the Knights Templar and the Zetas.