Mexican authorities discovered four human skulls wrapped in tin foil inside a cardboard package headed to the United States on Sunday.

The Mexican National Guard says they found the package after it went through a security checkpoint at the Queretaro Intercontinental Airport. The package came from the western Mexican state of Michoacan and was destined for Manning, South Carolina, Reuters reported.

Authorities have offered no further details regarding the identities, age or gender of the skulls.

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Michoacan is among the most violent areas in Mexico, seeing frequent incidents of cartel violence. Authorities have not stated whether they believe the four skulls are evidence of a crime, however.

Read more: Fox News