Sinn Fein has rubbished claims in a new book that Gerry Adams was an MI5 informer.
Undercover War: Britain's Special Forces and Their Battle Against the IRA, by Harry McCallion, makes the astonishing claim based on a letter made public in 2017.
The ex-SAS solider and RUC officer quotes from Irish state papers that mention how Fr Denis Faul put forward the theory that eight IRA members shot dead in 1987 by undercover troops at Loughgall were "set up by Gerry Adams himself".
Sinn Fein has consistently dismissed the cleric's claims as "utter nonsense".
Mr McCallion, who did several tours of Northern Ireland with the SAS before spending six years in the RUC, admits he wanted Adams "dead".
"This wasn't a sudden urge. I was a veteran with seven tours of Northern Ireland under my belt and I'd given the matter some thought," he confesses.
"One night in the mess bar, drinking with a serving officer from MI5, I was loudly voicing my opinion. Looking back, I must have sounded naive and spectacularly ill-informed, but I truly believed the best way to stop the terrorists was to target the high command. Like I said, kill Gerry Adams.
"The somewhat inebriated MI5 officer's response was surprising: 'No. He's one of ours.' I cannot confirm whether this was true, or whether it had its origins in the kind of drunken bravado that leads to all sorts of tall tales in the mess.
"However, the look of shock on the officer's face after the words came out, and his refusal to continue the conversation, were suggestive, as was the fact that he was unwilling ever to speak with me again outside a formal setting.
"I've never forgotten that night in the mess. Looking back from today's perspective, many of the most secret and dangerous operations undertaken by British forces in the province, and their outcomes, make more sense to me if the British security services truly did have an informer right at the top of the Republican movement."
Mr McCallion is adamant that a senior IRA informant tipped off the authorities about the Provos' planned attack on Loughgall RUC station, which led to the gang's biggest loss of life during the Troubles.
He suggests Adams could have leaked the information - which the ex-Sinn Fein president denies - to advance the party's political ambitions.
In the years after Loughgall the IRA suffered further heavy casualties in east Tyrone, killings that severely weakened one of its most active and deadly units.
McCallion adds: "That information devastated the East Tyrone brigade, and it kept coming. It seems quite coincidental that so much of this intelligence happened to be directed against figures in the PIRA who had the potential to threaten Adams' path towards a settlement."