The murder of two Royal Ulster Constabulary officers in 1997 led newly elected British prime minister Tony Blair to doubt whether he could deal with Sinn Féin, at a time when he was working to bring the party to peace talks.
Mr Blair was left "angry, disillusioned and betrayed" after the murder of John Graham and David Johnston, who were both shot in the back of the head while they were on patrol in Lurgan in June 1997. They were the last RUC officers to be killed by the IRA during the Troubles.
In a letter released on Tuesday by the UK National Archives, Mr Blair's private secretary, John Holmes, said shortly after the event: "The prime minister has severe doubts, in light of the double murder, whether serious business can be done with Sinn Féin."
At the time, the British government’s position was that Sinn Féin’s entry into talks was dependent on an IRA ceasefire. The murders resulted in contacts between the British government and Sinn Féin being severed.
Then taoiseach, John Bruton, saw the killings as a deliberate attempt to intimidate the governments into making concessions, so that the violence would stop, wrote Mr Holmes. Either Sinn Féin was acting in bad faith, or they were not able to "deliver their troops", Mr Bruton argued.
The North Armagh brigade of the IRA later claimed responsibility for the deaths. However, Gerry Adams initially tried to suggest the murders were the work of extremists and that "since the struggle was ongoing, these things were bound to happen" according to a phone call he had with John Hume.
In the days prior to the shootings, civil servants were discussing how their central objective was to give Sinn Féin one further chance to join the talks process. There was international condemnation of the murders and Mr Bruton and Mr Blair appeared together on television denouncing the deaths.
The two RUC officers, both community policemen, had been on foot patrol just before midday in Lurgan when two IRA men rushed behind them and shot them in the back of the head.
The IRA declared a ceasefire the following month. Mr Blair had put forward a timetable for the entry of Sinn Féin to talks for six weeks after the ceasefire was declared. In a report after the six-week period had ended, then secretary of state for Northern Ireland, Mo Mowlam, said IRA activity had been at a very low level and there was little evidence of targetings or punishment attacks.
Mr Holmes told the prime minister that the IRA’s adherence to the ceasefire had been “impressive and better than in 1994” when they had declared the first ceasefire.
“We have no real grounds for not now inviting them to the talks,” he said.