A Mexican Federal Police officer guards the access road to Puente Grande federal prison near Guadalajara, January 28, 2005. REUTERS/Bernardo De Niz

MEXICO CITY (Reuters) - Suspected drug gang hitmen dumped five severed human heads in ice coolers on a road in western Mexico on Tuesday with a message threatening rivals, a state attorney general’s office said.

Police patrolling a highway on the edge of the Mexican colonial city of Guadalajara found the heads inside five coolers left on the roadside, the Jalisco state attorney general’s office said.

“They were recently severed heads, cut off about four hours before they were found,” a spokesman said, adding police found a message left by apparent drug hitmen threatening rival gangs. The victims’ identities were not disclosed.

Torturing and beheading rivals is common in Mexico’s increasingly vicious drug war, which killed more than 6,000 people last year and shows no sign of abating this year despite the deployment of 45,000 troops across the country.

President Felipe Calderon has centered his presidency on curtailing drug war violence and reforming corrupt police forces working in league with traffickers, but the bloodshed is frightening investors and tourists from northern Mexico.

Jalisco state is close to the Pacific state of Sinaloa, home to major traffickers including Mexico’s most-wanted man, Joaquin “Shorty” Guzman, and a key transit point for drugs smuggled through Mexico into the United States.

The discovery of the heads of five men between the ages of 30 and 50 follows the killing of seven people by drug hitmen in February in a restaurant in Jalisco, which had been relatively quiet in the drug war over the past year.

Drug violence has spiked dramatically since Calderon took office in December 2006 and launched his army crackdown, but the government says the surge in killings shows cartels are being weakened by the offensive.