CIUDAD JUAREZ, Mexico — Police in northern Mexico have received an ice chest packed with four human heads.

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The chest that arrived at the Ascencion police station in Chihuahua state was marked "vaccines," but wasn't claimed for a week.

Finally, police opened it and found the heads. The bodies haven't been located.

State prosecutor's spokesman Eduardo Esparza said Tuesday the chest arrived on a local bus line that also has a delivery service. An envelope with an undisclosed message was found inside.

The heads appeared to be part of a wave of drug-related killings. Cartels have turned to beheading their enemies as they battle each other for control of lucrative smuggling routes.