The shooting dead of Denis Donaldson, once a key figure in Sinn Fein, highlights the dangerous world of security forces informers in Northern Ireland and how little safety there is for those who choose that route.
Denis Donaldson: Agent inside Sinn Fein
In Northern Ireland, where paramilitaries regard loyalty to a cause as more important than family, there is no greater crime than to break an allegiance with your own people and turn police informer.
For decades the IRA has dispensed its own form of brutal summary justice to those deemed to be "touts".
The loyalist way of dealing with informers has been no less violent.
But without a doubt, informers have been the crucial cog in the intelligence machine in Northern Ireland.
In the early days of the Troubles, the security forces had very poor, if virtually no intelligence, of what was going on inside some of the paramilitary groups.
During the first days of internment in the 1970s, most of the republicans held by the security forces were old-timers - the new leadership had mostly escaped.
But by the 1990s, the security apparatus was arguably the most sophisticated in the world, even if its methods, when exposed, provoked outrage among some.
Informers in Northern Ireland often found themselves in the middle of an intricate security web, sometimes answering to RUC (now PSNI) detectives, sometimes to the force's own independently-organised Special Branch.
But other agencies were there too. With Northern Ireland being the top security priority beyond the Cold War, MI5 had its people on the ground and was very often at the heart of secret meetings, or just message passing, between republicans and the government.
William Stobie: Special Branch informer, loyalist quartermaster
Eventually, the British Army established its own networks - both through military intelligence gathered by special forces soldiers, very often operating closely with other agencies, and in the shape of the ambiguously titled - and controversial - Force Research Unit, which recruited and ran informants.
The process of recruitment was long and required patience. Handlers would spend months identifying potential informers - then the target would be groomed and slowly brought into the fold.
The most important informers to the security forces were those who were prepared to remain inside their organisations and supply a constant stream of information that could be used to prevent bombings or shootings.
The authorities believed, and still do, that one good agent on the inside is worth dozens on the outside. Among those recruited on both sides - and now dead - were Denis Donaldson from Sinn Fein and William Stobie, a loyalist.
In the early 1980s, the security forces sought to make maximum capital out of informers through the "supergrass" system.
Some 30 supergrasses were recruited from the IRA, INLA and loyalist UVF.
These were supposed to be key figures, in terms of the information they knew, and the RUC would use their statements as the basis of charging more than 300 people with terrorist offences.
But despite the drama, the system ultimately failed as almost half of the supergrasses retracted statements - and many of those convicted were cleared on appeal.
While some of the supergrasses were promised a new life and immunity from prosecution, republicans claimed that others were intimidated into turning informers.
But it is the use of informers still inside paramilitary organisations that remains the most controversial.
The role of these informers had led to persistent allegations from nationalists of collusion between the security forces and loyalist paramilitaries - not least in the murder of Belfast solicitor Pat Finucane, a lawyer who had acted for senior republicans and IRA members.
Graffiti against Ken Barrett appeared in Belfast
In a number of special investigations in recent years, BBC journalists Peter Taylor and John Ware have detailed how fine the line had become between security forces running agents and abetting the operations of loyalist paramilitaries.
One of the army's key agents was Brian Nelson, a former soldier from Belfast. He became the UDA/UFF's head of intelligence, providing gunmen with the information they needed to identify targets.
The FRU's plan was to use Nelson to prevent the UFF killing ordinary Catholics and concentrate on targeting republicans.
But allegations persist that Nelson's handlers knew of planned operations - but did not do enough to stop some of them.
Nelson's murky role between paramilitaries and aiding the law was discovered by a 1990 investigation into collusion and he was jailed on five counts of conspiracy to murder.
Far from bringing the affair to an end, allegations persisted that members of the security forces had crossed the line between protecting an agent from discovery and preventing a killing taking place. And at each stage of the investigation, the role of informers became more and more clear.
William Stobie, a loyalist paramilitary quartermaster implicated in the Finucane killing, walked free from court in 2001 - only to be killed days later by his own former UDA comrades. He had been revealed to be working for Special Branch.
In turn, another UDA man, Ken Barrett, was secretly recorded by Panorama reporter John Ware - and also spoke to police officers investigating collusion between the Army's Force Research Unit and loyalist paramilitaries.
Barrett avoided the fate of others: as loyalist graffiti branded him a traitor, he fled and was later jailed after pleading guilty to his part in Pat Finucane murder.
What lies behind the April 2006 death of Denis Donaldson, a British agent for 20 years at the heart of Sinn Fein, is unclear.
But the violent end of men like Mr Donaldson - dying at an isolated County Donegal cottage - and William Stobie - killed outside his Belfast home - shows that the life of an agent once exposed is extremely risky.
They are caught between the world of paramilitaries that they were once part of, and that of the security forces who may never have fully trusted their catch.
In the extreme environment of Northern Ireland, the process of being exposed can lead the former agent with no idea whom to trust, or to whom to turn.
And that's the dilemma that applies to all informers.
In January 1999, former IRA man Eamon Collins became one of the highest profile republicans to be killed by former colleagues. He was battered to death in his hometown of Newry.
He had renounced the republican armed campaign, written a book, Killing Rage, detailing the violence of the IRA, and appeared in court defending the Sunday Times newspaper in a libel action against an alleged senior IRA man.
For all of this, he was regarded by former IRA colleagues as an informer and traitor. Despite death threats, he chose not to spend his life on the run.