Four human skulls were discovered inside a package at a Mexican airport that was due to be sent by courier to the United States, local authorities said on Friday.

The skulls were found wrapped in aluminum foil inside a cardboard box at Queretaro Intercontinental Airport in central Mexico, a statement from the National Guard said.

The package had been sent from the western coastal state of Michoacan – one of the most violent parts of the country – and was destined for an address in Manning, South Carolina, it said.

The National Guard gave no further details on the age, identity or possible motive for the sending of the human remains.

The transfer of human remains requires a special permit from a competent health authority which was not obtained, it said.