Two men and a woman have been arrested over the murder of Northern Ireland prison officer David Black.
Mr Black, 52, was shot as he drove to work on a motorway in Co Armagh on November 1. Dissident republicans claimed responsibility.
The father of two worked at the high security Maghaberry jail in Co Antrim.
A 36-year-old man was arrested in Co Tyrone by the Police Service of Northern Ireland and taken to Antrim police station for questioning.
Meanwhile, in the Irish Republic, a man aged 44 and a woman aged 31 have been detained by gardai.
The pair were being held under section 30 of the Offences of the State Act 1939 at Ballymun Garda station.
Mr Black was the first prison officer murdered in Northern Ireland for 20 years.