A cooler containing the heads of four men has been delivered to police in Mexico.
The grisly discovery was made by a messenger who took delivery of the parcel at the main police station in Ascension, near the violent city of Ciudad Juarez, on the U.S. border.
The container was marked "sensitive," and police at first believed the shipment was meant to be delivered to a hospital.
Officers said the heads were from men aged between 25 and 35 and could belong to any of 10 local people who were kidnapped last week by gunmen.
Police are working to identify the heads and inform next of kin.
Ciudad Juarez is said to be the most dangerous city in the country, with more than 1,000 of Mexico's 3,800 drug-linked killings taking place there since the start of the year.
More than 200,000 white-clad demonstrators took to the streets earlier this year to protest over rising violent crime.
More people have been killed this year than in all of 2007 with Mexico overtaking Colombia and Iraq with its kidnapping record.
However, numbers could be much higher as many rights groups say two or three more kidnappings are committed for each one reported.