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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Skulls Mexico

Four human skulls wrapped in plastic and aluminum foil are seen during an inspection by the National Guard inside a package bound for the United States at a courier company located at the Queretaro Intercontinental airport, in Queretaro, Mexico.  (Mexico's Guardia Nacional/Handout via REUTERS)

Skulls mexico us

Four human skulls wrapped in plastic and aluminum foil are detected by X-ray equipment during an inspection by the National Guard. (Mexico's Guardia Nacional/Handout via REUTERS)

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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.

Mexico's government has struggled to combat drug cartels and human traffickers flowing across its borders and into the U.S.

Migrants at border

Migrants attempting to cross in to the U.S. from Mexico are detained by U.S. Customs and Border Protection at the border August 20, 2022 in San Luis, Arizona.  (Nick Ut/Getty Images)

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President Biden's administration is also struggling to stem the flow of migrants and drugs at the U.S.-Mexico border, which saw record-breaking migrant encounters throughout last year.