By Egmont R Koch and Nina Svensson
Directors, Underworld Art Deal
For almost a decade two Turner masterpieces disappeared from public view into the hands of criminal gangs. Eventually they were recovered but only after the Tate had handed over several million pounds to a well-placed intermediary.
The missing Turner masterpieces were painted in 1843
Did the Tate pull off a remarkable rescue or did they in effect pay a ransom and set a dangerous precedent that makes future art theft more likely?
In July 1994 two of the Tate's Turner paintings - "Shade and Darkness" and "Light and Colour" - were stolen while on loan to the Schirn Kunsthalle in Frankfurt.
A leading figure in Frankfurt's Balkan mafia was believed to be the brains behind the theft, but only the two actual thieves and a handler were ever convicted.
But exactly six years after the robbery, the first painting was returned to London, followed by the second 18 months later.
So what happened during those first six years to guide the paintings home again?
At the time of the robbery, each painting had been insured for £12m and the Tate put in its claim for £24m.
If the masterpieces were in the hands of the Balkan mafia, this was to be no easy undertaking
But this also meant that if the stolen Turners were ever found, the paintings would belong to the insurance company.
Dissatisfied with this arrangement, the Tate took a gamble.
In July 1998 they paid back £8m to the insurance company for the ownership rights and would later seek permission from the High Court in the UK to spend £3m of the Turner Bequest fund to try and recover the paintings.
If the masterpieces were indeed in the hands of the Balkan mafia, this would be no easy undertaking.
German lawyer Edgar Liebrucks had clients in the Balkan mafia
Two retired policemen who had previously worked on the case for Scotland Yard - Detective Superintendent Mick Lawrence and Detective Sergeant Jurek Rokoszynski (known as Rocky) - came on board as private detectives to help the Tate reclaim two of Britain's national treasures.
The end game began in the summer of 1999, when the Metropolitan Police received confidential intelligence suggesting that a German solicitor may be able to assist in the recovery of the paintings.
The man's name was Edgar Liebrucks and he said he was in direct contact with the people who were in possession of the paintings.
He signed a contract with the gallery, but also asked the Frankfurt public prosecutor's office for legal immunity and, according to Liebrucks, this was granted.
"This made it possible for me to talk to people in the certainty that I wouldn't be called upon as a witness in a trial and made to tell a court what was discussed," he says.
The Turner case became a personal challenge for Mick and Rocky
This mission was a big culture shock for Lawrence and Rocky.
"I'd never been in a situation like this," says Lawrence, "where the recovery of the property was far more important than the arrest of the people who were in possession of the property."
The operation was extremely delicate.
"Edgar was working on behalf, I suppose you could say, on behalf of the criminals, but he was also working for the Tate," says Lawrence, "whereas we were working solely for the Tate. At times the situations became blurred."
But in July 2000, Liebrucks recovered the first Turner painting.
The sum of DM5m (£1.7m) had been paid by the Tate, plus an extra DM300,000 (£100,000) for Liebrucks' services.
Sandy Nairne, Tate's Director of Programmes 1994-2002, inspected the second Turner in Frankfurt
The second Turner was more difficult to retrieve.
One day Liebrucks got a visit from a second gang, who said they had the picture.
Were they simply fall guys for the Balkan mafia worried about the police closing in on them?
"They were very amateurish," the lawyer recalls, "not like violent criminals."
But the deal with them did materialise and a couple of days before Christmas 2002, Rocky was driven to a flat outside Frankfurt where he could inspect the painting.
Photographs of it were taken and forwarded to the Tate's expert in London, before money (£1.7m) was made available to Rocky for Liebrucks' clients.
The private home, where the photographs were taken, held the key to the whole story.
The man who lived there led us to the two men who had possession of the second Turner.
One of them was Josef Stohl, who had hidden the masterpieces for the Balkan mafia behind spare car parts.
Stohl's business affairs were in a mess, and his mate Hartmut Klatt came up with a big but very dangerous idea: they could make a fortune by selling the Turner Stohl was guarding for the mafia and keeping the money for themselves.
They sold the second Turner painting to Liebrucks and ran away from the mafia with the money, first to Cuba and then to Brazil.
The masterpieces were returned to the walls of the Tate.
But had the gallery actually bought the Turners back from the Frankfurt underworld, effectively giving money to criminals?
Or, as the gallery insist, did they only pay for the crucial information that led them to the paintings' whereabouts?
The Tate rigorously defends the actions it took to recover the missing Turners.
A gallery spokeswoman insists that any money handed over was for information and that "no ransom was paid".
She also told the BBC: "The Tate acted throughout the investigation with the assistance and advice of the Metropolitan Police and dealt with a reputable German lawyer."
Sandy Nairne, former Director of Programmes at the Tate who oversaw the operation, also emphasises the Tate's working relationship with the appropriate authorities.
He said: "I think what we knew in all the different stages of investigation was that a reward would be necessary... but it only emerged rather later that there might be a particular kind of discussion through intermediaries, and that discussion could only take place with the approval of the various authorities. That meant authorities both in London and in Germany.
He added: "They (the paintings) belong to the public and they should be seen by the public."
Underworld Art Deal was broadcast on Wednesday, 9 November, 2005 at 1900 GMT on BBC Two.