Efforts to trace £57m-worth of Chinese artefacts stolen from a Cambridge University museum five years ago have proved fruitless, police said.
Thieves broke into the Fitzwilliam Museum on 13 April 2012 and escaped with 18 mainly jade items but since then there has been no trace of them.
Despite the passage of time, the museum remains hopeful of their return.
But an art expert believes the objects have been sold into China and could take generations to resurface.
A number of people were jailed for their roles in the Fitzwilliam robbery and other raids on museums and an auction house across the UK.
While items including a rhino head and Chinese artefacts were retrieved and returned, none of those from the Cambridge museum was ever found.
"Artwork is either recovered very quickly, or the thieves realise what they've got is radioactive, and it goes underground for a generation or more," Christopher Marinello, founder of Art Recovery International, said.
With the Fitzwilliam artefacts registered on a number of art databases including Interpol and Artive, any dealer exercising due diligence would realise the items are stolen "and that's how they might be located", he said.
Because the theft was so widely publicised, Mr Marinello believes the Fitzwilliam jade has "gone underground", most likely traded among criminals, perhaps for drugs or weapons.
While Cambridgeshire Police have confirmed the case is still open, the force is not looking for anyone else in connection with the theft.
The Fitzwilliam remains hopeful its jade will be found and returned, a spokeswoman said.
However, lawyer Mr Marinello, who specialises in recovering stolen artwork for museums, churches, insurance companies and private clients, thinks the museum could be waiting some time.
"I believe the Fitzwilliam jade has made its way to the top market for it in the world - and that's China," he said.
"I think they're in Chinese collections and until someone perhaps dies and the next generation decides to sell, I don't think we'll see them for quite a while."