Authorities at Chicago's O'Hare International Airport have held up a shipment of 18 human heads sent from Rome and bound for a facility in the U.S. where they would be used in anatomical research.

A spokeswoman for the Cook County Medical Examiner's Office, Mary Paleologos, says the heads were properly preserved, wrapped and tagged as human specimens and there is no suggestion of foul play.

It was not immediately clear why customs officials had held up the shipment.

The containers were being stored at the Cook County morgue until authorities decide what to do with the shipment.

Paleologos says the specimens were frozen in sealed containers and were sent from a legitimate facility in Rome about a week before Christmas.

She didn't have information on the U.S. facility.