Customs officials at O'Hare International Airport in the U.S. city of Chicago are holding a shipment of 18 human heads reportedly bound for a U.S. research facility.

The local medical examiner's office says the heads, sent from Rome, were properly preserved, packaged and tagged as human specimens. There is no suspicion of foul play.

The Chicago Tribune newspaper cites a spokesman for the medical examiner's office as saying authorities held up the shipment because the final destination was not clearly indicated on the accompanying paperwork.

U.S. Customs officials discovered the heads on Monday. They were delivered to the medical examiner's office for inspection and storage while officials investigated.