Dissident republican Colin Duffy has been arrested by murder squad detectives investigating the drive-by shooting of a prison officer in Northern Ireland.
He was detained with a second man in Lurgan, Co Armagh, just miles from where David Black, 52, was ambushed on the M1 motorway on his way to work at the top security Maghaberry Jail, near Lisburn, Co Antrim.
Detectives leading the inquiry insisted they needed the public's help to bring the killers to justice. Superintendent Keith Agnew declared: "Condemnation, however strident, is not enough. It needs to be translated into information if our investigation is to make maximum progress."
Duffy, 44, who has been cleared of murder charges on three separate previous occasions - the latest last January after two soldiers were shot dead outside Massereene Army barracks in March 2009 - was arrested at his home in the Kilwilkie estate where republicans opposed to the peace process have huge support.
Politicians on all sides condemned the murder of the prison officer, who was planning to retire next year after more than 30 years of service.
Enda Kenny, the Irish Republic's Prime Minister who was in Armagh for talks with Northern Ireland First Minister Peter Robinson and Deputy First Minister Martin McGuinness, said the dissident republicans had been linked to criminality and drug dealing which had also led to deaths on the streets of Dublin.
Mr Black's murder had overshadowed the meeting, he said. He added: "We do not want to see a return to this, nobody on this island wants to see a return to this."
Police confirmed the dark blue Toyota Camry car used by the gunman and then later burned out had Dublin-registered number plates.
Duffy and the second man, 31, are being held at Antrim, but detectives clearly believe it will be up to the public to give them the breakthrough they need.
Mr Black drove from his home in Cookstown, Co Tyrone, then through Stewartstown, and minutes after pulling on to the motorway in his Audi A4, the car carrying the gunman pulled up alongside. The killer opened fire after a passenger side window was lowered. Mr Black was hit, and his car then careered off the road and into ditch.