Dissident republican Colin Duffy has been arrested by police investigating the murder of a Northern Ireland prison officer, sources said.
He and a second man were detained by officers in Lurgan, Co Armagh, just miles from where David Black, 52, was assassinated in a drive-by shooting on the M1 motorway.
Duffy, 44, was acquitted by a judge in Belfast earlier this year of the murders of two soldiers shot dead by dissident republicans outside Massereene military barracks in Antrim in March 2009. The second man is aged 31. Both men were taken for questioning by detectives at Antrim.
Mr Black, from Cookstown, Co Tyrone, was shot dead as he drove to begin duties at Maghaberry Prison, near Lisburn, Co Antrim, on Thursday.
Prime Minister David Cameron and all sides in Northern Ireland condemned the shooting. It was also discussed today at a north south ministerial meeting in Armagh involving Enda Kenny, Taoiseach of the Irish Republic, Northern Ireland First Minister Peter Robinson and Deputy First Minister Martin McGuinness.
Duffy faced separate murder charges on two previous occasions and was cleared both times, once by the Court of Appeal.
Sinn Fein president Gerry Adams said the killing of the prison officer was wrong. There was no future in such actions, which were rejected by the entire community.
He added: "Whatever position individuals may hold on the efficacy or otherwise of armed struggle in the past, there is no rationale for it in the present circumstances. There may be a small number of people who tolerate the existence of militarist groups or their violent actions or who provide shelter, resources or facilities to them. They need to reflect on what they are doing. These groups are not the IRA and nobody should be under any illusion about that."
Arriving at a scheduled cross-border meeting with Mr Kenny, Mr Robinson gave his reaction to Duffy's arrest. He said: "I don't think it's right for us at this stage to be making any judgment. It will be for the police to put together a case and I really do encourage them to do that."
Sue McAllister, director-general of the Prison Service in Northern Ireland, met Mr Black's widow, Yvonne, at her home in Cookstown, Co Tyrone, today. Afterwards she told Wendy Austin on BBC Radio Ulster's Talkback programme: "The family is completely shattered. They are very close. It is a dreadful tragedy and we will make every effort to support them in whatever way we can."