By Edward Green

Missing - but for how long?

The recovery of stolen art is a long, careful process, often involving fairly unorthodox tactics, and is not always handled by the police.

Lessons for the recovery of The Scream may well be learnt from the past. Ten years ago, in February 1994, another version of the painting - there are four - was stolen from the national gallery in Oslo. Three months later, it was successfully retrieved, and returned to the gallery. In 1996, four men were arrested for the theft.

One of those responsible for the recovery was Charles Hill, at the time a member of the Metropolitan Police's art and antiques squad. He travelled to Oslo, impersonating an American art collector from the wealthy J Paul Getty Museum in California.

He successfully opened negotiations with the thieves, and in addition to retrieving the painting in a buy-back sting operation, gathered evidence which eventually convicted them.

Mr Hill believes the key to such a successful sting rests with the thieves themselves, who are just as keen as collectors to buy into the myths of the art world, and who accepted his disguise as a wealthy American without question.

"The art thief catches the disease, you see - there is a madness that afflicts these people", he told the Evening Standard last year.

Chief among the "diseases" is the idea that stolen art is commissioned by wealthy collectors, who are willing to risk arrest for the pleasure of owning a famous painting that they can never show to anyone.

The art thief catches the disease...there is a madness that afflicts these people Charles Hill, art retrieval expert

This is more the stuff of Bond films than of real life, says Tony Russell, who also participated in the retrieval of The Scream 10 years ago, and now works for a private art recovery firm, Art Retrieval Ltd.

"Having done 20-odd years in the recovery art business, I've never, ever come across that scenario anywhere in the world," he says.

So if they are unable to sell the painting, what might the thieves hope to gain from it? One possibility is that they might attempt to make money from it by ransoming it back, either to the gallery or to the Norwegian government.

�300,000 was demanded for the return of the version stolen in 1994 - a poor return, perhaps, on a painting valued at up to �30m.

But at the time the Norwegian government were adamant that no ransom would be paid, says Russell, and it seems unlikely that they will move from that position.

Another possibility is that the painting will be traded on to another group of criminals, possibly for drugs or for a fraction of its value.

For art retrieval experts, the ransom demand provides the best way of recovering a stolen work of art. This, however, can sometimes bring them into conflict with the police, who believe that they prefer to pursue a successful recovery rather than a successful conviction.

ART RECOVERIES 1911: The Mona Lisa was stolen from the Louvre. It was recovered two years later in Florence. 1991: 20 works of art, including Sunflowers, were taken from the Van Gogh Museum in Amsterdam, but found half an hour later. 1995: Titian's Rest on the Flight to Egypt was stolen from Longleat - it was recovered in 2002.

"We do feel that effectively buying back paintings could be encouraging more thefts," a senior source in the Met said last year.

The dangers of employing unorthodox tactics are illustrated by the subsequent ending to the 1994 story of the stolen Scream. The men convicted for the theft in 1996 were later freed - after Hill's evidence was ruled to be inadmissible since he had entered the country on a false passport.