A group calling itself the "IRA" said it was responsible for placing the car bomb which detonated in Londonderry on Jan. 19 in a statement sent to the Derry Journal yesterday.

Northern Irish police said the main focus of the investigation was on the New IRA (Irish Republican Army), one of a small number of groups opposed to a 1998 peace deal that largely ended three decades of violence in the British-run province. "We also caution those who collaborate with the British that they are to desist immediately as no more warnings will be given," the statement said.

The attack came amid uncertainty over the future of the Irish border as Brexit is adding to tensions in Northern Ireland. Brexit negotiations, which have raised the possibility of reinstating a hard border between Northern Ireland and the Republic of Ireland, had brought fears of a return of violence.

Two former police chiefs of Northern Ireland earlier warned about a new wave of terror arguing that dissident republicans aim to exploit any Brexit-related chaos. The warnings followed reports that British intelligence service MI5 has stationed more than 700 officers in Belfast as part of an intelligence operation to combat the growing threat and mainly the New IRA group.

Londonderry, known as Derry by its majority Irish nationalist population, is a city known for its violent past due to sectarian violence, known as The Troubles. More than 3,700 people died during decades of violence before the peace accord. The 1998 Good Friday Agreement put an end to decades of sectarian conflict in Northern Ireland, but tensions persist between the Protestant and Catholic communities. Most militants have renounced violence, but some IRA dissidents carry out occasional bombings and shootings.