A man arrested in connection with the murder of prison officer David Black in the North has been released unconditionally.
The 46-year-old from Lurgan, Co Armagh, was the fourth man detained and then released by detectives investigating the shooting.
Mr Black, a 52-year-old father of two from Cookstown, Co Tyrone, was gunned down in a high-speed motorway ambush as he drove to work at Maghaberry prison in Co Antrim last week.
The long-serving officer was nearing retirement.
Dissident republicans have been blamed for the attack.
The Crimestoppers charity has offered a £10,000 reward for information that leads to the conviction of Mr Black's killers.
At his funeral earlier this week his daughter Kyra described the prison officer as her special hero.