The severed heads of four men have been delivered by a courier service to a police station in northern Mexico.
The heads, all of men believed aged between 25-35, arrived last week in an icebox marked as containing vaccines.
Police thought the package was meant for a local hospital - and only opened it on Monday, revealing the four heads.
Police are investigating if the heads belong to any of 10 people who were kidnapped last week.
The gruesome delivery was made in the town of Ascension, not far from Ciudad Juarez, close to the US border.
The BBC reports Ciudad Juarez has a reputation as one of Mexico's most violent cities, with more than a quarter of the country's 3,800 drugs-related murders reported to have taken place there since the start of the year.
Authorities have discounted reports that one of the heads may have been of a police commander who was kidnapped on 18 May this year.