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, 43, 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 morning. The car the shooters used was believed to have been registered in Dublin.
Prime Minister David Cameron and all sides in Northern Ireland condemned the shooting.
It was also discussed at a north-south ministerial meeting in Armagh involving Taoiseach Enda Kenny, 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.