Denis Donaldson, who has been found shot dead in the Irish Republic, was once a key figure in Sinn Fein's rise as a political powerhouse in Northern Ireland.
Mr Donaldson (front) publicly admitted being a spy
But for years he also led a double life as an agent for the British intelligence services.
There will be little doubt that his death is connected to that confession.
For two decades, Mr Donaldson, who was in his mid-50s, was a key informant at the heart of Sinn Fein's political machinery.
He played this role during the crucial years when the republican movement began the slow shift away from an armed campaign, leading ultimately to the IRA's ceasefire and, most recently, dumping of arms.
Like many of his closest former confidantes, his ties to republicanism were formed in the early years of the Troubles when young men believed they needed to resort to violence to defend Catholic communities.
As with virtually every other senior member of Sinn Fein from the era, he was interned for periods during the 1970s.
Over the years, he became a trusted and respected senior Sinn Fein figure, close to both party president Gerry Adams and chief negotiator Martin McGuinness.
Following the Good Friday Agreement and the eventual devolution of power to Northern Ireland, Sinn Fein appointed Denis Donaldson as their key administrator in the party's Stormont offices.
It was a key role - co-ordinating the party's tactics and business in the troubled power-sharing arrangements.
But during all of this time, he was an agent passing information to the security services.
That double life was revealed in the fallout from the controversial - and some would say murky - circumstances of the police raid on Sinn Fein's Stormont offices in 2002, amid allegations of a republican spying ring at the heart of government.
That raid led to the suspension of power-sharing after unionists declared they could no longer trust Sinn Fein and threatened to quit.
Three men, one of them Mr Donaldson, were arrested at the time - but late in 2005 prosecutors said they were dropping all charges "in the public interest".
That public interest became clear when days later Mr Donaldson confessed publicly to being a double agent, an admission that led to his immediate expulsion from Sinn Fein and republican circles.
In his statement in December 2005, Mr Donaldson confirmed that he had been a British agent at the time of the raid and he apologised to his "former comrades" and family.
"I was recruited in the 1980s after compromising myself during a vulnerable time in my life," he said. "Since then, I have worked for British intelligence and the [Police] Special Branch.
"Over that period I was paid money. My last two contacts with Special Branch were as follows: two days before my arrest in October 2002, and last night, when a member of Special Branch contacted me to arrange a meeting.
"I was not involved in any republican spy ring at Stormont - I deeply regret my activities with British intelligence and Special Branch."
At the time, Gerry Adams claimed that Mr Donaldson had spoken out because he was about to be "outed" by police officers determined to deflect attention from the raid.
But the republican movement, secretive and often clan-like, was utterly shocked by the revelation.
Denis Donaldson disappeared from the Belfast political scene almost immediately after the revelations.
But a Dublin-based newspaper tracked him down to a tiny, run-down house in the hills of County Donegal's north-west coast.
Journalist Hugh Jordan of the Sunday World found Mr Donaldson living in squalid conditions in a virtually derelict cottage without running water or electricity.
He had no contact with the outside world and no police protection.
The newspaper found Mr Donaldson chopping logs outside the home and he told the journalist he just wanted to be left to live in peace. Mr Jordan said that Mr Donaldson appeared nervous and depressed.
Past informants - dubbed "touts" in republican circles - were usually tortured, interrogated and shot by the IRA.