A 22-year-old man was arrested in the North today following the murder of Constable Stephen Carroll by dissident republicans.
The police officer was shot dead by the Continuity IRA as he answered a call for help in Craigavon, County Armagh, in March last year.
The suspect was detained in the Newtownabbey area, near Belfast, and is being questioned at Antrim police station.
A teenager and a 38-year-old man are in custody charged with the murder.
Constable Carroll, 48, was married and came from Banbridge, Co Down.
He was killed two days after the Real IRA shot dead two soldiers outside Massereene Barracks in Antrim.