Mexican authorities have discovered the decapitated bodies of 12 men, including some believed to be soldiers, in the southern state of Guerrero.
Earlier media reported there were nine bodies found in and around the state capital, Chilpancingo.
Authorities said the heads of the men were found in plastic bags in a separate, nearby location.
Authorities found a sign near some of the bodies that read: "For every one of mine that you kill, I will kill 10."
Mexico is caught in a rising wave of violence as drug cartels battle for territory and fight a nationwide crackdown led by President Felipe Calderon, who has deployed tens of thousands of troops and federal police across Mexico to try to stop the violence.
Police say the cartels are battling for control of smuggling routes into the United States.
There have been nearly 5400 drug-related homicides across Mexico this year.
Some information for this report was provided by AP.