The decapitated bodies of 15 young men have been found in the Mexican beach resort of Acapulco.
Police said they were discovered near a shopping centre and were all aged between 15 and 25.
They were dumped there by drug cartel members fighting over the control of the drugs business in the city.
More than 30,000 people have been killed in drug-related violence in Mexico since late 2006.
President Felipe Calderon has deployed thousands of troops to battle the drug cartels, and claims to be making progress in reducing their influence.
But critics of Mr Calderon's policies say they have increased the level of violence without reducing the flow of cocaine and other drugs into the US.
Human rights groups have also raised concerns that using the military has exposed civilians to possible abuse.
In this latest violent incident, police responding to a report of a burning vehicle near Acapulco's Plaza Senderos shopping centre shortly after midnight found five other abandoned vehicles and the decapitated bodies.
Media reports say that three messages signed by Joaquin "El Chapo" Guzman, leader of the Sinaloa cartel and Mexico's most wanted drug baron, were found alongside the bodies.
The messages reportedly said that the attack was a warning against criminal groups fighting Mr Guzman's organisation.
It is believed to be the largest single group of decapitation victims found since President Calderon launched his offensive against the cartels.
Acapulco is in an area of Mexico where the drug trade is controlled by La Familia Michoacana, one of the most violent cartels.
The BBC's Julian Miglierini says the city, which became a popular summer resort in the 1970s and 1980s, has seen a sharp rise in violence related to drug gangs.
He adds that while the government insists that the city remains a safe destination for tourists, many fear that Acapulco's reputation had been badly damaged by the violence.