Three men are still being held in connection with the murder of prison officer David Black on Thursday.
Mr Black (52) was gunned down on Thursday during an ambush on a motorway in Co Armagh as he drove to work at Maghaberry high-security prison.
Two men, including prominent dissident republican Colin Duffy, were arrested on Friday morning in the Lurgan area. Both are being questioned by detectives from the PSNI serious crime branch at Antrim.
A 29-year-old suspect was arrested in Co Leitrim on Friday night and is being held at Carrick-On-Shannon Garda station under Section 30 Offences Against the State Act 1939.
US secretary of state Hillary Clinton yesterday branded the killing as a cowardly and senseless act.
"There is no justification for this outrageous and cowardly act," she said.
A funeral service for Mr Black is to take place on Tuesday.
The service will be held at Molesworth Presbyterian Church in Cookstown where the father-of-two had attended regularly.