Allegations of collusion between members of the security services and loyalist paramilitaries remain one of the most controversial episodes of Northern Ireland's conflict.
Pat Finucane: Family believed his death could have been stopped
And these events, even if the full details remain hidden, are still playing a role in the ongoing peace process.
But what exactly was collusion?
During the height of The Troubles, various branches of the security services in Northern Ireland, from MI5 and military intelligence officers through to some parts of the then Royal Ulster Constabulary, devoted huge resources into trying to penetrate paramilitary organisations.
Their stated aim was to infiltrate armed groups such as the IRA or loyalists organisations such as the Ulster Defence Association to stop their activities and ultimately prevent terrorist attacks and deaths.
But in 2003, Sir John (now Lord) Stevens, the then Metropolitan Police Commissioner, concluded a major investigation into collusion.
It found that rogue elements within the police and army helped loyalist paramilitaries to murder Catholics in the late 1980s.
A key military intelligence file from 1973, entitled Subversion in the UDR, estimated that five to 15% of Ulster Defence Regiment soldiers were linked to loyalist groups.
The document added that the "best single source of weaopns, and only significant source of modern weapons for Protestant groups, has been the UDR.
The best-known case investigated by the Stevens Inquiry was the 1989 murder of Belfast solicitor Pat Finucane. Mr Finucane was a defence lawyer who had represented in court a number of alleged members of the IRA.
Like most other respected criminal lawyers in the city, Protestant and Catholic, some of his clients were paramilitaries.
An army intelligence team called the Force Research Unit was running a loyalist paramilitary double-agent called Brian Nelson.
Brian Nelson: Loyalist paramilitary, British agent
Nelson compiled a dossier on Mr Finucane that in turn led to loyalist gunmen shooting the solicitor 14 times as he sat at the family dinner table. His wife and three children witnessed the murder.
The question is how much did Nelson's army handlers know? For years, Mr Finucane's family said there was sufficient evidence to prove that his death could have been prevented.
A subsequent BBC Panorama investigation alleged that a Special Branch officer in the then Royal Ulster Constabulary persuaded loyalists to kill Mr Finucane.
ROYAL ULSTER CONSTABULARY
Former NI police force
Targeted by republican groups
302 officers killed
Supported by unionists
Replaced by new PSNI force
Some officers linked to collusion
Critically, he concluded that Mr Finucane's death, and that of another Catholic man, could have been averted.
Lord Stevens said informants and agents "were allowed to operate without effective control and to participate in terrorist crimes" because their handlers believed the running of these agents was more important than the lives of those who were threatened.
In political terms, collusion is a major issue that goes beyond questions of prosecuting those alleged to have taken part.
Republicans are currently debating whether or not to finally back the Police Service of Northern Ireland, the reformed force that replaced the Royal Ulster Constabulary.
And so the story of collusion will play a crucial role in whether trust can be built up between the PSNI and some nationalist communities.