The skulls were boxed and wrapped in aluminum foil, apparently to avoid airport scanners. The package came from the state of Michoacan, one of the most violent states in the country, and was on its way to Manning, South Carolina.
The National Guard has not released any information about the age, identity or possible motive for sending the skulls. Human remains cannot be sent just like that, but must be accompanied by a special permit. In this case it was not present.