Officers found the decapitated bodies of nine men in an abandoned pickup truck on a highway in the drug-plagued Mexican state of Guerrero, police said Friday.

The state Public Safety Department said the bodies and severed heads were inside 18 plastic bags left in the bed of the truck, which was found blocking a highway in the town of Tlapehuala late Thursday.

The department said in a statement that police also found a threatening message attributed to the drug cartel known as La Familia.

Guerrero is experiencing a wave of violent crime that authorities say is part of a battle between La Familia and other gangs for control of drug routes in the Pacific coast state.

Also Friday, the navy enacted new rules prohibiting sailors from shooting at vehicles that try to evade land checkpoints unless they are fired on or feel that their lives or others' are in danger.

Navy ground forces have played an increasing role in combating the cartels, but operate far fewer checkpoints than the army.

The army, which has been criticized for incidents in which civilians were shot at roadside outposts or checkpoints, did not immediately respond to requests for comment on its rules for similar situations.

Mexico has come under increasing pressure to make progress in improving human rights compliance by the armed forces under the Merida Initiative, a $1.4 billion, three-year aid package from Washington. The U.S. Congress must withhold some of the money unless the State Department reports that Mexico is not violating human rights in the drug war.

More than 13,800 people have died in unprecedented drug violence in Mexico since President Felipe Calderon launched a nationwide crackdown on cartels in late 2006.