"This will happen to everyone that helps the traitor Edgar Valdes," the placard read, referring to a leading drug capo whose real name is Edgar Valdez.
It was signed CPS - the initials for the South Pacific Cartel, a relatively new drug gang that has claimed responsibility for other gruesome killings.
Drug violence has escalated across Mexico as president Felipe Calderon goes after powerful cartels and as rival gangs fight over turf.
More than 28,000 people have died in drug violence since Calderon took office in late 2006.
While much of the bloodshed is centered in northern Mexico, violence has climbed Cuernavaca, a once-quiet city, since security forces killed a top drug trafficker - Arturo Beltran Leyva, in a gunfight in December.
His death kicked off a power struggle within his cartel, and rival gangs have been seeking to co-opt his territory.
Valdez, a Mexican-American known as 'La Barbie' due to his blond hair, is a leading contender to head the Beltran Leyva cartel.
A spokeswoman for the state attorney general's office said the crime appeared to be linked to drug trafficking and that federal authorities would lead the investigation.