On 19 December 2004, masked raiders took £26.5m in cash from the vaults of Belfast’s Northern Bank, loaded it on to a truck and vanished into the night.

It was one of the most audacious heists in British criminal history, and it left an enduring riddle: did the IRA do it? Police on both sides of the Irish border suspected so, but the group denied any involvement. The robbers have not been caught and the investigation remains open.

Now a former IRA volunteer and convicted bank robber, Ricky O’Rawe, has added to the intrigue by writing a novel with echoes of the raid.

Northern Heist, published on Friday, is a rollicking, colourful account of the preparation and execution of a very similar robbery. The fictional one is committed by a Belfast criminal gang headed by a character named James “Ructions” O’Hare, a professional thief. The IRA is not involved in the robbery but demands a cut of the proceeds, leading to twists and double-crosses.

Facebook Twitter Pinterest Cover of Northern Heist novel. Photograph: Mal McCann

Given O’Rawe’s background – he was part of an IRA unit that robbed a Belfast bank in 1977 and was an IRA press officer during the 1981 hunger strikes – some wonder how much of Northern Heist is fiction.

Publicity for the book, published by Merrion Press, describes the author as a “writer with the inside knowledge, literally and metaphorically, to deliver the goods”, and says he blends factual events with “vivid and authentic fictional characters”.

Interviewed this week at his west Belfast home, O’Rawe, 64, said he believed the IRA did in fact rob the Northern Bank. “I think the IRA done it,” he said. “I’m not sure anyone other than them could have pulled off something of that magnitude. I’d be fairly confident it was the IRA.”

He claimed to have no inside information. “I don’t know who did it. There are gangs in Belfast who are master thieves and who are potentially capable.”

In the absence of certainty, the novel posited its own narrative, said O’Rawe. “There is no real evidence to say the Provos did rob it. So I said to myself: ‘What if they didn’t? What if it was the work of one man, a criminal mastermind?’”

Big criminal operations in Northern Ireland usually set tongues wagging, but the Northern Bank heist was different, demonstrating discipline among those responsible, said O’Rawe. “Not one murmur. I’ve never seen anything as tight as this bank robbery.”

Posing as police officers, masked and armed men entered the homes of two Northern Bank executives and held their families hostage while the executives gave the gang access to the bank’s underground vaults. The raid yielded one of the UK’s biggest cash hauls and jolted the peace process. Police speculated that it was intended to provide pensions to IRA members.

There have been 20 arrests but only one conviction, that of Ted Cunningham, a financial adviser from Cork who was given a five-year suspended sentence in 2014 for laundering some of the cash. The robbers’ identities remain unknown.

O’Rawe said his own bank robbery – £11,000 taken from a Northern Bank branch on the outskirts of Belfast in 1977, for which he served six years in jail – was a traditional stick-up job. Holding bank staff relatives’ hostage – a so-called tiger kidnapping – was cowardly, he said. “It should not be romanticised, it’s pure intimidation.”

O’Rawe, who originally titled the novel Greed, has previously written three non-fiction books: a biography of Gerry Conlon, a wrongly jailed member of the Guildford Four, and two about the hunger strikes. He has accused Gerry Adams of rejecting a deal with British authorities that could have saved hunger strikers’ lives, a claim the former Sinn Féin leader has denied.

O’Rawe said three film companies, including a BBC affiliate, were exploring options to turn Northern Heist into a film. He plans to write a sequel, possibly set in Paris or London.