The case of John Downey goes to the very heart of the understanding between the government and the IRA which finally closed the lid on the Troubles.
he Old Bailey ruling yesterday demonstrates how Sinn Fein can look after its own, by providing security for former IRA militants who, like Mr Downey, took the path of peace rather than joining the ranks of the dissidents.
Mr Downey's release speaks to the credibility of both the government and Sinn Fein. It showed how effective the republican party's negotiators were, and that the government could be held to its word – even when it agreed something that would turn the stomach of many.
Mr Justice Sweeney summed this up when he dismissed the case against Mr Downey.
He said the public interest in prosecuting someone for mass murder on the streets of London was "very significantly outweighed" by the public interest in "holding officials of the state to promises they have made in the full understanding of what is involved in the bargain".
But what exactly was the bargain that the courts ordered the government to honour? Why exactly is John Downey so important? And, most pressingly of all, does it mean that victims of Troubles-era terrorism can forget about any prospect of justice if the killers of their loved ones hold a 'letter of comfort' from the Northern Ireland Office?
The treatment of offenders from the Troubles era was always difficult. The initial deal in the Good Friday Agreement was that all prisoners who gave allegiance to a paramilitary group on a verifiable ceasefire would be released within two years.
The first extension to that deal was that anyone caught for a terrorist offence that happened before the signing of the 1998 Agreement would serve a maximum sentence of two years.
Next, jail breakers who had already served two years before escaping were told they could call it quits and come home.
But there still remained a small cadre of nearly 200 suspects, mainly belonging to the IRA, who were on the run. Known as OTRs, they remained outside the UK because they feared arrest and a two-year sentence if they came home.
Tony Blair promised to find a way to accommodate them.
In 2005, he put forward legislation which would have allowed a special tribunal to make a finding of guilt and immediately release them on licence.
But this was opposed by unionists, and Sinn Fein swung against it when they realised former members of the security forces, such as soldiers who killed innocent civilians on Bloody Sunday, were also eligible to apply to the tribunals.
The idea was dropped and OTRs were instead advised to write to the Northern Ireland Office, which would tell them if there was evidence on which they could be arrested.
A few found they were wanted but most – 187 – learnt that, whatever they were suspected of, there was no evidence against them.
John Downey, as close as you can get to republican royalty, was one of them.
From Creeslough in Donegal, and with a conviction for IRA membership, he was believed by police to have taken part in IRA bombings in England in the 1980s.
His fellow suspects included Eibhlin Glenholmes, once Britain's 'most wanted' woman; Brighton Bomber Pat Magee; Martina Anderson, now an MEP, and, inevitably, at least one informer, Sean O'Callaghan.
They were regarded as something of an A-Team who took the war to Margaret Thatcher. Mr Downey's mystique grew when he turned up in Donegal as London police named him as one of their most wanted. He had been here all along, signing on the dole, he said.
A year later, the police in Northern Ireland suspected him of involvement in the Enniskillen poppy day massacre.
He had a reputation for extreme violence and ruthlessness, the personal courage to take risks and the ability to keep one step ahead of the authorities. This was just the sort of person whose support Sinn Fein needed as it moved towards peace and a deal with the British – and that support was forthcoming.
At home in Donegal, he threw his organisational skills into Sinn Fein, where he remains a member. He was crucial to the campaign of Pearse Doherty to become a Sinn Fein TD in Donegal South. Mr Doherty, a young man with no IRA involvement, needed the street cred and energy of a republican veteran like John Downey to get the vote out and stand at his side on doorsteps. It was in 2007, when Mr Doherty won a landslide election victory at the age of just 30, that Mr Downey chanced his arm and wrote to the NIO to see if he was possibly wanted by the authorities.
He must have been pleasantly surprised by the reply when it came back: "There are no warrants in existence, nor are you wanted in Northern Ireland for arrest, questioning or charging by police. The Police Service of Northern Ireland is not aware of any interest in you by any other police force."
Mr Downey started travelling, first to Canada in 2008 with his wife, where he was granted a temporary residence permit.
A year later, he visited Belfast and Londonderry to promote greater understanding between republican and loyalist ex-prisoners.
He visited the UK seven times without hindrance before he was arrested at Gatwick Airport on his way to Greece in May last year.
There was predictable outrage from Sinn Fein, Pearse Doherty ran fundraisers and visited him, while Gerry Kelly spoke of bad faith on the part of the British.
But was it an accident? The PSNI is taking the blame for supplying false information to the NIO and ignoring a 30-year-old warrant from the Met which had apparently been waiting patiently to catch Mr Downey off guard for all those years.
One senior officer involved in authorising the 'letters of assurance' said that initially, the police checked for warrants and evidence here.
Although they found suspicions about Downey, there was no evidence to hold him on.
The NIO then asked if other forces were pursuing Mr Downey.
But the Met warrant, which was on PSNI records, was somehow overlooked, perhaps by a more junior officer.
Dr Michael Maguire, the Police Ombudsman, has to sort out what went wrong, and the PSNI will review other letters sent to OTRs to find out whether any were sent in error.
It is acutely embarrassing for Sir Hugh Orde – the Chief Constable at the time and a stickler for procedure – particularly as he now sets police standards as president of the Association of Chief Police Officers.
The political repercussions are harder to gauge, but already, the DUP and Sinn Fein are at each other's throats and victims' groups are questioning whether there is any justice left for them.