Authorities at Chicago's O'Hare International Airport are holding up a shipment of 18 human heads meant to be used in anatomical research at a facility in the Chicago suburbs, local media reported.

The heads arrived from Rome just before Christmas, and officials initially said an issue with paperwork caused the hold-up, according to the Chicago Sun-Times. But the newspaper reported Tuesday that authorities stopped the shipment from going through because it was headed to a facility that is under investigation.

According to an anonymous source quoted by the Sun-Times, the investigation itself is “absolutely not” connected to the shipment of the heads.

A spokeswoman for the Cook County Medical Examiner's Office, Mary Paleologos, told The Associated Press the heads were now being held in containers at the Cook County morgue.

The heads, shipped from a legitimate facility in Italy, were properly embalmed, wrapped and tagged as human specimens, Paleologos told the AP. There is no suggestion of foul play, she added.

The heads, still covered in skin, were found Monday at the airport.

“There’s no issue with the transportation of body parts for medical purposes,” Brian Bell, a spokesman with the U.S. Department of Homeland Security, told the Sun-times. “There’s nothing against the law that says you cannot ship them, provided you have the right documentation.

“Everybody here is ‘Oh my gosh, you got a box of heads’ and everybody thinks that it’s unheard of,” Bell added. “It is a potentially legitimate medical shipment. We’ve seen it at various ports in the nation.”