Dissident Irish republican Colin Duffy is one of two men arrested this morning by officers investigating the killing a Northern Ireland prison officer David Black.
Duffy, 43, and a second man were detained by officers in Lurgan, Co Armagh, just miles from where Mr Black, 52, was assassinated in a drive-by shooting on the M1 motorway.
He 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.
Duffy faced separate murder charges on two previous occasions and was cleared both times, once by the Court of Appeal.
The arrested men have been taken to the PSNI's serious crime suite in Antrim.
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 is no rationale now for the existence of armed groups or for carrying out armed actions in any part of this country. Those involved have no popular support or political strategy.
"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.
"Those organisations that are politically associated with armed groups have failed to outline how these actions advance republican objectives.
"On the contrary, they play into the hands of those in the British system who are opposed to the peace process and to its potential for achieving a united Ireland."
Married father-of-two, Mr Black, 52, was shot several times from a car that pulled up alongside his on the M1 near Lurgan, Co Armagh, as he drove to work at Maghaberry jail yesterday morning.
The long-standing member of the Orange Order became the 30th prison officer to be murdered in Northern Ireland since 1974, though the first for almost 20 years.
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.
Colleagues said Mr Black, from Cookstown, Co Tyrone, had been actively considering retirement after more than 30 years' service.
Prime Minister David Cameron joined political leaders on both sides of the Irish border in condemning what he said was a "brutal murder".
He said: "These killers will not succeed in denying the people of Northern Ireland the peaceful, shared future they so desperately want."
Stormont First Minister Peter Robinson branded the culprits "flat-earth fanatics living in the dark ages, spewing out hatred from every pore".
After being shot, Mr Black's black Audi A4 veered off the road and crashed into a deep drainage ditch.
Police have blamed dissident republicans opposed to the peace process.
The violent extremists have been engaged in a long-running protest campaign against conditions inside HMP Maghaberry in Co Antrim - Northern Ireland's only maximum-security prison.
Ministers from the Northern Ireland Executive and the Irish Republic's government will discuss the murder at a North South Ministerial Council meeting in Armagh today, according to the BBC.
Stormont Deputy First Minister Martin McGuinness insisted the murder would not destabilise the peace process.
"Our community stands absolutely four-square and united against the activities of these groups," he said.
Mr Black has become the 30th prison officer killed in Northern Ireland since 1974, though the first for almost 20 years.
He was driving on the motorway between Portadown and Lurgan at about 7.30am when a dark blue Toyota Camry, with a Dublin registration, pulled alongside and several shots were fired.
Police Service of Northern Ireland (PSNI) Assistant Chief Constable Drew Harris said dissidents had been actively targeting prison officers.
He indicated the gunshots, not the crash, had been the cause of death, adding: "Mr Black appears to have sustained very serious and probably fatal gunshot wounds. The motive behind this is sheer terror."
The Toyota believed to have been used in the attack - registration 94 D 50997 - was later found burnt-out in the Inglewood area of Lurgan, Co Armagh - a town with strong pockets of dissident support.
Mr Black's service stretched back as far as the 1981 IRA hunger strike inside the Maze prison when 10 republicans starved themselves to death.
PSNI Chief Constable Matt Baggott said it was a "completely senseless attack" that "demonstrated the recklessness and ruthlessness and sheer dangerousness of those who oppose peace and are dedicated to taking us back to those dark days of the past".
Mr Black was a long-standing member of the Orange Order in Cookstown.
Grand Master of the Grand Orange Lodge of Ireland Edward Stevenson said he was the 337th member of the organisation to be murdered by terrorists since 1969.
"His professionalism throughout the worst of the Troubles and beyond is in stark contrast to the cowardly and faceless terrorists who today have left a wife without her husband and two children without their father," he said.
"Our thoughts and prayers are with David's wife, Yvonne, his children, Kyle and Kyra, and wider family circle at this deeply traumatic time.
"They can be assured that the Orange fraternity will rally around them in their hour of need."
Prison Service director-general Sue McAllister said Mr Black had expressed interest in an early retirement scheme but his departure date had not been set.
She vowed the officer's colleagues would not be bowed by the attack.
Northern Ireland Secretary Theresa Villiers branded the attack on Mr Black "cowardly and evil".
"Like his colleagues across the Prison Service, he was dedicated to serving the whole community in Northern Ireland," she said.
"This is in stark contrast to the people responsible for this despicable crime."
Irish Prime Minister, Taoiseach Enda Kenny, said the murder was "deeply disturbing".
"I utterly condemn the actions of those who carried it out and their scant regard for human life," he said while on an official visit to Berlin.
Mr Kenny added: "Those who committed this brutal act will rightly be condemned by all civilised and right-thinking people on this island who utterly reject such hideous and mindless violence."