Authorities at a Mexican airport made a gruesome discovery inside a package headed for the United States on Friday. Four human skulls were found wrapped in aluminum foil inside a cardboard box at Queretaro Intercontinental Airport in central Mexico, Reuters reported. The parcel was sent from Michoacan in western Mexico to an address in Manning, South Carolina. Authorities did not release information on the age of the skulls, or the motive for sending them. However, they added that whoever sent the package did not obtain a special permit required for sending human remains.