It is a beautiful crime scene. On the first floor of the Isabella Stewart Gardner Museum in Boston, the Dutch Room is lined with green silk wallpaper from its terracotta cobbled floor to oak-timbered ceiling. On the walls hang works by Rubens, Dürer, Van Dyke and others. But it’s the empty frames that catch the eye.

There’s an easel on which once stood The Concert, a masterpiece by Johannes Vermeer. It is now the world’s most valuable missing work of art, worth an estimated $200m. On the far wall, a gilded rectangle frames only silk wallpaper.

It once held Christ in a Storm on the Sea of Galilee, Rembrandt’s only known seascape. Like the Vermeer, it was cut from its frame in 1990 and disappeared, along with two other Rembrandts, five Degas sketches, a Manet painting, a landscape by Govert Flinck and a bronze finial from a Napoleonic battle flag. In all, around half a billion dollars worth of art.

The theft, in the early hours of 18 March – as St Patrick’s Day parade celebrations were winding down in the city – was executed by two men dressed as police officers who, after handcuffing museum guards, spent an almost leisurely 81 minutes in the galleries. It remains not only the biggest art heist of all time but also the largest single theft of private property in America. Despite a $10m reward, none of the works have been seen in public again.

Facebook Twitter Pinterest Rembrandt’s only known seascape, Christ in a Storm on the Sea of Galilee, was part of the haul from Isabella Stewart Gardner Museum. The total worth of the art stolen was estimated to be around $500m. Photograph: Alamy

One name that has always been associated with the heist is James “Whitey” Bulger. The former mob boss was killed in prison last week while serving two life sentences for 11 murders. From the early 1970s, Bulger headed the Winter Hill Gang, an Irish-American mob that terrorised Boston for more than a decade. But he lived a double life as an FBI informant, feeding the bureau information about rival criminals.

After going on the run in 1995, Bulger spent 16 years on the FBI’s most wanted list, topping it briefly after Osama bin Laden’s death in 2011. That year he was arrested while living in a seafront apartment in Santa Monica with his longtime partner Catherine Greig. The elderly couple had been hiding in plain sight and were known to neighbours as “the Gaskos”. FBI agents found a stash of $800,000 and an arsenal of 30 firearms. Art lovers were disappointed there wasn’t a Vermeer above the bed or a Rembrandt in the sitting room. Not even a Degas sketch or two in the bathroom.

After his arrest, Bulger did not volunteer information about the Gardner heist that might have brought a more lenient sentence or a more comfortable cell. And yet, according to Charles Hill, a former Scotland Yard detective turned private investigator, Bulger was the key to the theft.

“On the new morning of 18 March 1990, even the dogs in the streets of south Boston must have known that Whitey was involved in some way before, during, or after the robbery,” says Hill. “Whitey was an IRA sympathiser, he loved to associate himself with ‘the cause’, and was involved in arms deals and drugs shipments to the Republic.”

Hill believes the paintings were shipped to Ireland as part of a deal with an IRA-affiliated gang. “After a shipment of weapons and ammunition was intercepted by the Irish navy off the coast of County Kerry in 1984, Whitey felt he owed one to his friends in the Republic. I believe he offered them the paintings.

Facebook Twitter Pinterest Prime suspect: James ‘Whitey’ Bulger, who died in prison last week, was head of Boston’s Winter Hill Gang. Photograph: AP

“There is no hard evidence for this but I combat art crime both rationally and irrationally, intellectually and viscerally,” he says. “That technique serves me well as a style and measure of success.”

Hill’s track record is impressive. As an undercover detective, he led the 1996 operation to recover Edvard Munch’s The Scream, stolen two years earlier from the National Museum of Norway. In 1993 he led the recovery of a Vermeer and a Goya stolen in 1986 from Russborough House in County Wicklow. That theft was masterminded by Martin Cahill, a Dublin gangster known as the General, a nickname which gave title to the 1998 John Boorman film in which Brendan Gleeson played the crime boss.

According to Hill, the latter heist – with the Vermeer being the prize asset – was the inspiration for the Boston job.

Following a network of leads, many from underworld contacts, Hill is convinced that the Gardner treasures are still stashed in the Republic of Ireland. “Even if Bulger did not order the robbery originally, he would have muscled in and taken control of the haul soon after it took place.”

Anthony Amore, security director at the Gardner Museum, discounts Bulger’s possible involvement, citing lack of firm evidence. Amore, who spends a lot of time staring at the empty frames, says: “If you were a homicide detective, you’d go to the scene and see the taped outline of the person on the floor. I come in here every day and these are my taped impressions.”

John Wilson presents Front Row on BBC Radio 4