LONDON—Stodgy law offices and stolen masterpieces don’t often go hand in hand. But on October 4, Scottish police officers raided a corporate law firm in Glasgow, breaking up a meeting where an alleged deal was in the works over one of the world’s most valuable paintings.

The police operation was the culmination of a four-year search for Leonardo da Vinci’s "The Madonna of the Yarnwinder," which was seized in 2003 from the Scottish castle of the Duke of Buccleuch in one of the most audacious art robberies in years. The 1501 oil on panel, which had been listed on the FBI’s top-10 list of stolen artworks, is reportedly valued at approximately $60 million.

"We had the suspects on our radar for some time," says Detective Superintendent Brian Anderson of the Dumfries and Galloway Constabulary, the police force in southwest Scotland. "There were no surprises to the raid—we expected to find the painting, which we discovered in a carrier bag in the office. Still, it was a very exciting day."

To date, five men have been arrested in connection with the theft, with a total of four being charged, including Calum Jones, one of Scotland’s top insolvency lawyers. Jones, a corporate partner at HBJ Gateley, the raided law firm, has been charged with conspiracy to rob and to extort money. Another one of the men, Marshall Ronald, is described by Anderson as a "rogue lawyer" who was investigated six years ago in a fraud and banking scam. "There were a large group of people involved in a number of clearly defined phases of the theft," says Anderson. "Those that committed the robbery, supported it, stored the painting, handled the painting and tried to facilitate a deal for profit with either the Duke or the insurance company."

For Julian Radcliffe, director of the Art Loss Register, the world’s largest database for stolen art and antiques, the arrest of two lawyers in connection with the case is telling evidence of a trend in which deals involving stolen art often also include the assistance of unscrupulous legal professionals as intermediaries.

"We have two current cases, one in the U.S. and one in the U.K., where lawyers tried to negotiate on behalf of fences," notes Radcliffe. By selling art on the black market, thieves can only expect to see a small portion of its market value, and unloading world-famous masterpieces is nearly impossible, says Radcliffe.

So where is the profit? Generally, experts point to two scenarios: either a quick ransom or using the art as collateral in the criminal world— a kind of underground currency for drug deals and other illicit transactions.