Three men have been arrested in connection with the murder of prison officer David Black in Co Armagh yesterday.
A 29-year-old man was arrested in Leitrim at 5pm this afternoon and is being held at Carrick-on-Shannon Garda Station.
Earlier two men, aged 31 and 44, were arrested in the Lurgan area and taken to Antrim for questioning.
One of the men arrested in Lurgan has been named as Colin Duffy, a prominent Republican activist.
Mr Black, 52, was shot on the M1 motorway as he drove to work at Maghaberry high-security prison.
Meanwhile, detectives have appealed for information about a Dublin-registered Toyota Camry used by the killers.
The dark blue car had a registration plate 94 D 5099.
Detective Superintendent Keith Agnew, who is leading the murder inquiry, has also appealed for the public's help in relation to Mr Black's car.
Police want to hear from anyone who saw his Audi A4, registration number HHZ 1560, at any stage during the drive from Cookstown, through Stewartstown, onto the Tamnamore roundabout and eastbound onto the M1.
Superintendent Agnew said this afternoon that there was "a long way to go" in this murder investigation.
Mr Black had over 30 years' service in the prison service and was nearing retirement.
There has been widespread condemnation of the killing.
The Taoiseach has pledged the full support of the Government and that of gardaí into the investigation of the murder.
Arriving at a meeting of the North-South Ministerial Council in Armagh this morning, Enda Kenny said any assistance required by the PSNI would be "immediately forthcoming".
Mr Kenny described the murder as "a very sad case" and said he could guarantee that gardaí would work "closely" with the PSNI both in terms of "intelligence and information".
He said he hoped the perpetrators of the murder, "those who planned it, who ordered it and carried it out", would be brought to justice.
Tánaiste Eamon Gilmore said that what took place yesterday was "truly shocking".
He said the "appalling" attack on Mr Black had been condemned by every "right-thinking person on the island".
Mr Gilmore said his immediate concern was for the dead man's family. He said his sympathy goes out to them.
Northern Ireland Secretary Theresa Villiers said that authorities in Northern Ireland and Britain are more determined than ever to fight the threat of dissident republicans.
Speaking on RTÉ's Morning Ireland, Ms Villiers said that despite what had been achieved in the past 20 years, there are still those who had "lethal intent".
Chairman of the Prison Officers' Association of Northern Ireland Finlay Spratt said that prison officers have been more conscious of their security recently.