Customs officials made a gruesome discovery at Chicago's O'Hare International Airport -- 18 frozen human heads.
The heads were found during a routine X-ray of cargo that was passing through the facility, The Associated Press reported.
Police said the heads had been used for medical research and were being returned to Illinois for cremation.
But a glitch in paperwork held them up as U.S. Customs and Border Patrol agents investigated, ABC News reported.
The somewhat grisly discovery is not that uncommon.
Doctors, medical facilities and research companies ship human body parts all the time, officials told ABC News, but this time the shipment raised flags due to the paperwork problem.