RTÉ's Richard Dowling profiles the key players in the Smithwick Tribunal
The work of the Smithwick Tribunal has focused on what gardaí knew about the ambush in which the two RUC officers died and if there was a leak of information which allowed the IRA time to set up the attack.
At its core is the question of whether "members of An Garda Síochána or other employees of the State colluded in the fatal shootings of RUC Chief Superintendent Harry Breen and RUC Superintendent Robert Buchanan on the 20th March, 1989."
As a result of years of private and public investigation, the role of three former gardaí - and the force itself - were seen as crucial to answering that question.
Owen Corrigan: The now retired Detective Garda spent most of his career in Dundalk and retired at the rank of sergeant.
He was named in the House of Parliament by MP Jeffrey Donaldson as the mole who passed information to the IRA which led to the ambush and murder of the two RUC officers.
He has always strenuously denied the allegation and won damages in a High Court case when he was named in a newspaper article as the IRA mole. He was mentioned in an RUC intelligence form as being a person who had passed information to the IRA but again denied those claims in the witness box
Of the three former gardaí at the centre of the Tribunal's investigation he was questioned far longer than the others. Over several weeks of on-off appearances (due to ill health) he was asked about his work and his finances.
Other areas in which he was questioned on was how a man linked to the IRA threatened a witness in a case where Mr Corrigan was being prosecuted.
As a result the case never went ahead.
He was also asked about his abduction by the IRA in Drogheda several years after he retired. He, along with another man, were held for two days and assaulted before being released. He never made a complaint to gardaí about the incident.
Mr Corrigan has always strenuously denied any collusion with the IRA. He has always maintained that he fought them at every opportunity and his work led to many arrests.
He was commended on several occasions for his work by his superiors.
An RUC officer claimed that Mr Corrigan saved his life when they believed they were being followed by the IRA.
He was never arrested nor questioned about the allegations he was an IRA mole.
Leo Colton: Leo Colton reached the rank of sergeant when he retired from the force. He was questioned by gardaí after another sergeant, Finbarr Hickey, was arrested for stamping false passport applications, some of which ended up in the hands of the IRA.
Senior gardai who investigated the case told the Tribunal that they believed Mr Hickey when he said he was asked to sign the forms by Mr Colton. The claims were denied by Mr Colton and he was never charged in relation to the matter.
When he was in the witness box, Mr Colton was asked about his employers after he had retired and their links to republicanism. Mr Colton has also always denied any collusion with the IRA.
Finbarr Hickey: He's the youngest of the three gardaí investigated by the Tribunal. He is the only one of them to actually have been arrested, charged and convicted of an offence.
In 1996 he was a sergeant in Hackballscross when he signed a number of passport forms, including one for a wanted IRA member.
He said he didn't realise this and was only doing it as a favour for Leo Colton - who denied the claims.
Mr Hickey was arrested in 1998 and sentenced in 2001 to one year on four charges of forging passport application forms. In January, 2011 he received a suspended prison sentence on a charge cultivating cannabis at his home.
During the Smithwick Tribunal, evidence was given by the PSNI that an RUC intelligence form said Mr Hickey was supplying information to the IRA. He has strenuously denied the allegation. He was never arrested or charged about that allegation.
An Garda Síochána: The issue for the Smithwick Tribunal is whether the Garda Síochána could have done something differently or could have acted on information or intelligence that could have prevented the ambush.