On a Wednesday afternoon earlier this month, lots 94 and 95 went under the hammer at Bonhams auction house in New Bond Street, London. The items, a 1,600-year-old Attic jar and a Greek jug from 350BC, were listed for sale at between £2,000 and £4,000. Only one of them sold. Lot 95, the jug, intricately decorated to resemble a man's head, fetched £3,600.
But the buyer pulled out after learning what distinguished lots 94 and 95 from the rest of the 436 items under the hammer that day: both are believed to have passed through the hands of Giacomo Medici, an Italian art dealer who ran one of the world's biggest antiquities trafficking networks.
Medici was arrested in 1995, convicted in 2004 and sentenced to 10 years in prison, reduced to eight after his first appeal. He remains free pending a final appeal.
Medici sold looted antiquities into the open market with the help of contacts in the art world. After his arrest police found a file in his premises in Geneva whose very name etches anguish across the faces of auctioneers around the globe: the Medici Dossier.
The dossier contains some 20,000 Polaroid pictures of antiquities thought to have passed through Medici's hands. Among them are images which bear an uncanny resemblance to lots 94 and 95 from the sale on 6 October.
Selling goods once owned by a notorious art thief would undoubtedly sour the reputation of Bonhams, one of the most reputable auction houses in the world. But Bonhams was aware of the potential criminal link between lots 94 and 95. Days before the auction the house received an email from an eminent academic alerting them to the questionable provenance of the lots, but it pressed ahead with the sale.
The Italian authorities refuse to share the Medici Dossier with auction houses or the Art Loss Register, the international body which authenticates ownership of works of art. This makes it difficult to remove items for sale even if they are cited as being pictured in the dossier, potentially leaving British auction houses open to the accusation that they are dealing in stolen goods.
"The onus is on Bonhams or any auction house to do a due diligence test and make sure that they are not selling anything that could have come to them via illicit activity," said Dr David Gill, a reader in Mediterranean archaeology at Swansea University and the person who suggested to Bonhams that lots 94 and 95 in this month's sale were from the dossier.
In their defence, auction houses say that they cannot be expected to withdraw an item which has been authenticated by the Art Loss Register – to do so would be breaking a contract with the seller – on the say-so of someone who has seen parts of a dossier to which the auction houses have no access.
While the 6 October sale is the latest to be hit by suggestions of Medici loot being sold at auction, it is not the first. Bonhams withdrew an item from a sale in April this year after being told it appeared in the Medici files. Other auction houses have had similar problems.
Bonhams is reluctant to discuss the issue. Experts there are hopeful of persuading the Italian authorities to share the dossier – and fear that a public statement expressing their dissatisfaction with the current situation could scupper those discussions.
But a statement released ahead of the 6 October sale explained the difficult situation the auction houses find themselves in. Spokesman Julian Roup said that they could not withdraw the items because neither appeared on any stolen art databases, and that the so-called Medici Dossier "does not appear on any of the checkable databases".
Christie's in London is not known to have listed for sale any items from the Medici Dossier, but there was a suggestion in June this year that three items listed for sale in New York were also depicted in the Medici Dossier. But, with no access to the Medici files, Christie's refused to withdraw the items.
A spokesman said: "Christie's works closely with international authorities and organisations towards our shared objective of preventing the illicit trade in improperly exported or stolen works of art. We would encourage any person or organisation who is aware of improperly exported or stolen works of art to report the matter to the authorities and to register details of the work with the appropriate bodies."
Sotheby's too has listed items linked to the Medici Dossier, but that was in the 1980s. Sotheby's in London stopped trading in antiquities in 1997, partly due to the controversy surrounding the provenance of items.
All of the auction houses work in conjunction with the Art Loss Register (ALR), which maintains a database of all stolen or looted artworks in the world. But, as many items in the Medici Dossier were looted – taken after being discovered in the ground, which is illegal in Italy – many of the items had no previous owner to report them missing, nor, in many cases, was it even known that they existed.
The Italian authorities also refuse to share the dossier with the ALR. These two issues mean that while the auction houses will check the ALR before sale, none of the items in the Medici Dossier are on it.
Chris Marinello, an executive director of the Art Loss Register, explained: "The controversy is that the Italian government does not want to release the photographs. I can understand their reticence. The Italians know that there were other looters working with Medici and may want to convict some of them. Those pictures are evidence and they could argue that releasing them could prejudice future proceedings.
"That said, if they did release the dossier, it would allow many people – buyers, sellers and auction houses – to scrutinise the items before sale.
"There is no doubt that the Italian authorities could do themselves a great service by releasing more of this material to organisations like ourselves so we can take a better look at items that are being offered for sale and ascertain whether or not there is a legitimate argument for saying they have questionable provenance.
"I can certainly understand their reluctance to do so, but releasing some information would solve problems when auction houses do not know what to do."
But while figures in the art world say it is up to the Italian police to release the dossier, some argue that when even the slightest doubt is cast on the provenance of an antiquity, the auction house should remove it.
In London it is the job of Scotland Yard's Arts and Antiques Unit to investigate the sale of illicit antiquities. The Metropolitan Police refused to comment on the Medici Dossier. But Vernon Rapley, the former head of the unit, said that he too sympathised with the Italians' position. "Some of the items pictured in that dossier might have been legitimately owned. The fact that a dealer sells five stolen objects does not mean that everything in his collection was stolen. So it would be incorrect of the Italians to share that information without being able to say it is all definitely stolen. While I empathise with the views of the auction houses who want clarity, I think it is impossible for the Italians to give them the entire document."