A group calling itself the New IRA has claimed a bombing which killed prison officer Adrian Ismay.
It is a small but deadly organisation which has previously been linked to the killing of a police officer and a prison warder.
With the centenary of the Easter Rising approaching later this month, senior police officers have expressed concern dissidents opposed to the peace process may attempt to hijack the anniversary by launching another attack.
Police Service of Northern Ireland (PSNI) new recruit Ronan Kerr, 25, died in a booby-trap car bomb in Omagh in Co Tyrone in 2011. Prison officer David Black was shot dead as he drove to work on the M1 in Co Armagh in 2012.
While the actions of the violent extremists remain sporadic and the number involved low, this month’s attack in east Belfast has provided another stark reminder of their capacity and intent. Police believe the threat posed to members of the security forces in Northern Ireland is severe.
The New IRA is a dissident group which opposes Sinn Fein and its peace strategy.
It is said to consist of individuals from Omagh, Coalisland and the Toomebridge and Ballyronan areas along the shore of Lough Neagh, and in counties Monaghan and Louth in the Republic of Ireland, with their actions directed from Belfast.
In 2014, Gavin Coyle was sentenced to 10 years’ imprisonment for possession of explosives and firearms with intent to endanger life and membership of a proscribed organisation - the IRA.
Coyle was arrested as part of the investigation into the murder of Constable Kerr.
Police had raided industrial units near Coalisland in Co Tyrone and found one of the largest caches of weapons and explosives in Northern Ireland for many years.
The New IRA also claimed it murdered Northern Ireland prison officer Mr Black. He was driving to work at Maghaberry Prison, Northern Ireland’s high-security jail, when he was attacked and killed.
It followed a dispute at the prison, where a number of dissident inmates were held.
MI5, responsible for national security in Northern Ireland, has said dissidents seek to destabilise Northern Ireland through the tactical use of violence, targeting members of the police service and other security personnel as well as trying to cause disruption and economic damage.
“They have very little public support,” the security service added.
It said: “However, although security force pressure is constraining the threat and all dissident republican groups are under pressure, some attacks continue to get through.
“There were 16 national security attacks in 2015 and the threat to life posed by dissident republicans persists.”
Police have not confirmed whether Semtex explosive was used in the attack but the group’s statement claimed it contained a quantity of Semtex and a commercial detonator.
Semtex was supplied by Libya to the Provisional IRA in the 1980s during the Troubles.
The virtually odourless plastic material is relatively easy to use because it is stable, unlike home-made bombs.
It became a key component in the home-made bombs and mines, typically forming a small part of a larger device with sophisticated timers and detonators.
General John de Chastelain, head of the international body charged with decommissioning paramilitary arms in Northern Ireland, has said the IRA had put its weapons beyond use as part of the peace process.
But critics have questioned how dissidents managed to obtain the explosive.