Four human skulls were found in a package at Queretaro Airport in Mexico.
Package destined for South Carolina in the US originated in Michoacan, Mexico.
Details of skulls' identities, age, or gender were not released.
Authorities have not stated if skulls are evidence of a crime.
Michoacan state is known for drug trafficking and cartel violence.
Mexico faces the issue of drug cartels and human traffickers crossing borders into the US.
Biden administration addressing the influx of migrants at the southern border.
Record numbers of migrant encounters at the US-Mexico border in 2020.