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(USA TODAY) - U.S. Customs officials at Chicago's O'Hare International have held up a shipment of 18 human heads from Rome until they get proper paperwork with the intended destination.

The embalmed heads, tagged as human specimens, were packed in three coolers shipped on a Lufthansa flight just before Christmas, according to a spokesman for the medical examiner's office, the Chicago Tribune reports.

No foul play is suspected. The specimens, still covered in skin, appear to be intended for medical research.

The Tribune reported that the shipment was held up because the final destination was not clearly noted on all the documentation.

Customs then X-rayed the coolers and determined that they contained 18 human heads. One cooler was opened for a preliminary inspection, the medical examiner's spokesman said, according to the Tribune.

The Chicago Sun-Times quoted Brian Bell, a Chicago-based spokesman for the Department of Homeland Security, as saying, "We need to make sure that they are truly used for medical research purposes."

The medical examiner's office said the recipient of the shipment has not yet come forward, the Tribune reported.