A mob boss killed in prison last week was key to the notorious Isabella Stewart Gardner Museum theft, a private investigator has speculated.

The heist, which took place in Boston in the early hours of 18 March 1990, just as St Patrick’s Day parades were ending, is the largest unsolved art theft on record. Now former Scotland Yard detective, Charles Hill, says a murdered mobster and IRA sympathiser was its mastermind.

James “Whitey” Bulger led the Irish-American Winter Hill Gang from the early 1970s in a campaign of terror against Boston. Hill believes Bulger orchestrated the art heist and shipped the stolen paintings to Ireland to make reparations to an IRA-affiliated gang after a delivery of weapons and ammunition was intercepted by the Irish navy in 1984.

Two men dressed as police officers entered the Gardner museum on the morning of 18 March on the pretence of investigating a disturbance. They handcuffed the museum guards and spent 81 minutes cutting masterpieces by Rembrandt (‘Christ in a Storm on the Sea of Galilee’), Vermeer (‘The Concert’), Degas, Manet and others from their frames and escaping with them. The haul was worth around half a billion dollars and it has inspired countless theories as to its disappearance.

“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,” Hill said, “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”.

Hill’s theory is similar to that of Dutch investigator and art advisor, Arthur Brand, who also thinks the stolen paintings might be in the possession of the IRA. Brand claims to have spoken to former members of the IRA who said it was “common knowledge” the looted works were probably being held by the organisation.

Hill’s glittering track record for recovering stolen art is not to be sniffed at. He found a missing Vermeer and a Goya stolen from Russborough House in County Wicklow in 1993 and Edvard Munch’s ‘The Scream’ after it vanished from the National Museum of Norway in 1994. The County Wicklow theft was said to have inspired the Gardner heist.

Security director at the Gardner Museum, Anthony Amore, remains unconvinced by the Bulger theory, given the lack of firm evidence but Hill is the first to admit his methods are unorthodox:

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