Police are continuing to question three men about a bomb attack on a prison officer in Belfast last week.
The 52-year-old officer was seriously injured when a bomb partially exploded under his van on Friday morning.
The men who are 34, 39 and 45 were arrested along with a 34-year-old woman in the Belfast area on Sunday. The woman has since be released.
A dissident republican paramilitary group, widely referred to as the new IRA, admitted the bombing.
It said it targeted the victim because he was involved in training officers at Maghaberry Prison near Lisburn in County Antrim.
He is in a stable condition in hospital.