This article from MSNBC tries to attribute the finding of eleven headless bodies to drug gangs, but the following makes it clear that something else is at work.
11 headless bodies found near Mexico graveyardAll the bodies had star signs and the letter Z tattooed on them, police sources said. A 12th beheaded body was found 50 miles away in a small town to the east of Merida
The Z tattoo is the best evidence for the truth here. Clearly, these were military research experiments, tattooed with the letter Z by the Army to designate them as part of the Zombie project. They escaped from the underground research facility in Mexico and had to be dealt with swiftly by removing their heads (often faster and less messy than destroying the brain, see "Shaun of the Dead" for more). Then, a cover story was created about drug gangs.