Five human heads have been found inside coolers along a highway in western Mexico.

Jalisco state Public Safety Secretary Luis Carlos Najera says threatening messages aimed at drug traffickers were found with the heads, which were covered with tape and discovered in individual coolers near the community of Ixtlahuacan del Rio.

The state prosecutor's office said in a news release Tuesday that authorities are still searching for the bodies.

Investigators believe the victims were beheaded hours before police made the discovery.

Drug violence has claimed more than 7,200 lives since January 2008 as cartels battle each other and the government for territory. The bloodshed has become increasingly gruesome, with cartel hit men sometimes beheading victims or dissolving the bodies in caustic substances.