Three men have now been arrested in connection with the murder of prison officer David Black, who was shot as he travelled to work on the M1.
A 29-year-old man was arrested in Leitrim at 5pm yesterday evening and brought to Carrick-on-Shannon Garda Station.
The Herald understands he is a low-level member of the Real IRA and is being detained for withholding information.
Earlier two men, aged 31 and 44, were arrested in the Lurgan area and taken to Antrim for questioning.
One of the men is Colin Duffy (44), a top Republican dissident who gave the graveside oration at Alan Ryan's funeral.
The murder of married father-of-two Mr Black is the first terrorist killing of a prison officer in almost 20 years.
The 52-year-old was shot on the M1 as he drove to work at Maghaberry prison.
Mr Black had more than 30 years' service in the prison service and was nearing retirement.
Specialist garda units are investigating if the dissident republican gang behind the assassination was given logistical support by a Dublin RIRA faction.
There has been widespread condemnation of the killing.
Taoiseach Enda Kenny has pledged the full support of the Government and gardai into the investigation.
Mr Kenny said he hoped the perpetrators of the murder, "those who planned it, who ordered it and carried it out", would be brought to justice.
Tanaiste 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 went out to them.
Chairman of the Prison Officers' Association of Northern Ireland Finlay Spratt said that prison officers have been more conscious of their security recently.