Belfast, Jan 29 - A group calling itself the "IRA" said it was responsible for placing the car bomb which detonated in Londonderry in a statement sent to the Derry Journal on Tuesday.
Northern Irish police said the main focus of the investigation was on the New IRA - 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 to the Journal said.
No one was injured in the blast outside a courthouse on Jan. 19 but the incident highlighted the threat still posed by militant groups opposed to the peace agreement.
Police in Northern Ireland and European Union member Ireland have said that a return to a hard border between the two after Brtain leaves the EU, complete with customs and other checks, could see a return to strife.
In the statement the group denied that Brexit was a motivating factor for their actions.
"All this talk of Brexit, hard borders, soft borders, has no bearing on our actions and the IRA won't be going anywhere."
"Our fight goes on," it said.The IRA was the principal nationalist paramilitary group during the decades of violence between Protestant Unionists and mainly Catholic Republicans until it agreed to a ceasefire in 1994.