MEXICO CITY (AP) — Four human skulls wrapped in plastic and aluminum foil and destined for South Carolina were found Friday at an airport in central Mexico, authorities said.

Mexico’s National Guard said its officers found the skulls at an airport in Queretaro after an X-ray detected strange shapes in a cardboard box. The box was being sent from the southern city of Apaztingan in Michoacan state to an address in Manning, South Carolina.

The Guard said the shipment, regardless of how the skulls were obtained or whether the items were meant for medical study, may violate Mexican laws on the handling of corpses.

Apatzingan has long been dominated by the Viagras drug cartel.