A group calling itself the New IRA has claimed responsibility for a Semtex bomb attack that injured a prison officer in Belfast, according to reports.
The 52-year-old, a married father of three, required surgery after an explosive device detonated under the van he was driving on Friday morning.
Four people including a woman have since been arrested.
In a statement to the BBC the dissident republican group reportedly said the officer was targeted because he was involved in training other guards at HMP Maghaberry, near Lisburn.
A spokesman said the officer was one of a number on a list of potential targets and the attack arose from a dispute over the treatment of dissident Republican inmates.
The group claimed to have used the plastic explosive Semtex and a commercial detonator in the attack.
Following the blast, police commanders expressed fears that it could be the first of a number of dissident republican attacks to mark the centenary of the Easter Rising.
In separate incidents on Saturday, officers found two viable explosive devices in residential streets in west Belfast.
The blast happened in the Hillsborough Drive area off Woodstock Road, a predominantly loyalist area in the east of the city, just after 7am on Friday.
The victim, a long-serving officer based at Hydebank Wood Young Offenders Centre in south Belfast who works as a trainer for new recruits to the NI Prison Service, had just left home to drive to work. His condition has been described as stable.
A spokesman for the Police Service of Northern Ireland (PSNI) said on Sunday: "Detectives investigating the attempted murder of a prison officer in Belfast on Friday 4 March have arrested four people: three men aged 34, 41 and 45 and a female aged 34.
"They were detained in the Belfast area this evening. They are currently being questioned at a police station in Belfast."