Gardaí last night released a man they had been questioning in connection with the murder of prison officer David Black.
The 29-year-old man was released from custody at Carrick-on-Shannon station in Co Leitrim yesterday evening.
Two men being questioned by the PSNI were also released without charge in recent days.
Mr Black, a married father-of-two, was gunned down by suspected dissident republicans during last Thursday's high-speed motorway ambush as he drove to work at the high-security Maghaberry prison in Co Antrim.
Thousands of mourners are expected today at the funeral of Mr Black, the first prison officer murdered by paramilitaries in the North for almost 20 years.
Cookstown in Mid Ulster will come to a standstill as former colleagues, friends and neighbours pay their respects.
The security forces in the North are still on high alert amid fears of further attacks from gunmen opposed to the peace process; even as the threat level from Irish terrorism in Great Britain was recently downgraded from substantial to moderate by the British Home Office.
Mr Black's children, Kyle and Kyra, will pay tribute to their father during today's ceremony at Molesworth Presbyterian Church in the Co Tyrone market town.
Family minister the Rev Tom Greer, head of the Presbyterian Church Dr Roy Patton and prison chaplain Rev Rodney Cameron will also take part in the ceremony, a spokesman for the Presbyterian Church said.
Mr Black was a long-standing member of the Orange Order and many of his friends from that organisation will also be present.
Sinn Féin MP for the area and Stormont Deputy First Minister Martin McGuinness will not attend after his offer was declined by the family, it is understood.
First Minister Peter Robinson will be at the funeral with several other DUP ministers and MLAs.
Gunmen travelling in a stolen car fired on the victim's Audi car near a junction leading to Portadown, Co Armagh, as he was on his way to Maghaberry. The car careered off the road and into a ditch.
Mr Black, who had more than 30 years' service, was the first prison officer to die at the hands of paramilitaries since 1993.
His family appealed for no more violence in a statement issued through a clergyman in Cookstown.
Prior to the church service there will be a private family service at Mr Black's home. He will be buried at Kildress Parish churchyard.
Yesterday the Northern Ireland Assembly stood for a minute's silence in respect for the victim and several members paid tribute to him.