CHICAGO (AP) — It sounded ghoulish enough: a shipment of 18 frozen human heads discovered and seized by customs officials during routine X-ray screening of cargo arriving at O'Hare International Airport in Chicago.

Turns out the heads were used for medical research in Italy and were being returned for cremation in Illinois. The holdup was due to a paperwork problem.

It just so happens such shipments are commonplace, and heads — quite a few of them — crisscross the globe via airplane and delivery truck.

"Just last week, we transported eight heads, unembalmed, to Rush University Medical Centre for an ophthalmology programme," said Paul Dudek, vice president of the Anatomical Gift Association of Illinois, which supplies cadavers and body parts to medical schools in the state for training students.

His association sends about 450 whole cadavers to medical schools each year and also ships individual body parts, including about a dozen shipments of heads annually.

The heads are used for training in fields such as dentistry, ophthalmology and neurology, where they are used for Alzheimer's research. They are also used to train plastic surgeons and by students learning to perform facial reconstructions on accident and trauma victims, Dudek said.

Most cadavers are obtained through voluntary donation by people who designate a willingness to have their bodies benefit science upon their death, Dudek said. A much smaller proportion are the bodies of people whose families could not afford their burial and so agree to allow the state to release them for research.

The shipment to O'Hare was properly preserved, wrapped and labelled "human specimens," said Mary Paleologos, a spokeswoman for the Cook County Medical Examiner's Office, which took hold of the shipment on Monday for storage in its morgue cooler while authorities continued to investigate the paperwork.

With little information initially, news of the shipment's discovery fuelled headlines and raised questions about where the shipment came from, where it was headed and why.

In the end, it turned out the shipment of three containers, which arrived in mid-December, was held up because of a mix-up with the paperwork and there was nothing suspicious about it or its destination.

The heads were originally sent from Illinois to a medical research facility in Rome and were returned to the Chicago area for disposal as part of the agreement for the order, Paleologos said.

On Tuesday, a cremation service arrived at the Medical Examiner's Office with paperwork for the specimens. Once federal authorities confirm the paperwork, the specimens will be turned over to the cremation service, she said.

US Customs and Border Protection could not discuss the specific case because of privacy laws, but it said shipments of human remains into the US "are not without precedent," are lawful with the right documentation and fall within the agency's "low-risk" category.

Dudek said such shipments require thorough documentation, in part because the scarcity of bodies donated to science means there is a black market for them.