Northern Ireland police on Monday questioned four men over a car bomb linked to a dissident republican group that has heightened concerns about the risks of Brexit disrupting a hard-won peace in the British province.
The bomb detonated at 8:10 pm (2010 GMT) on Saturday outside the courthouse in the border city of Derry.
Police have said they believe a paramilitary group calling itself the "New IRA" was behind the blast.
"We haven t seen a device of this nature function for quite a while. It s a high-risk tactic," Mark Hamilton from the Police Service of Northern Ireland (PSNI) told BBC radio.
Hamilton said the New IRA has recently mainly been linked to vigilante incidents and the bombing was "probably the most significant attack in recent years".
A 1998 peace deal largely brought an end to three decades of sectarian bloodshed between republican and unionist paramilitaries, as well as British armed forces, in Northern Ireland in a period known as "the Troubles".
Some 3,500 people were killed in the conflict -- many at the hands of the Irish Republican Army (IRA).
Hamilton said on Sunday that the New IRA were "determined to drag people back to somewhere they don t want to be".
There are concerns the attack could be a sign of paramilitaries seeking to exploit the current political turbulence over the status of Northern Ireland and its border with the Republic of Ireland, caused by Brexit.
"Dissident bomb fuels fear of return to terror after Brexit", ran a headline on the Irish Independent on Monday.
"Incidents such as the Derry bombing remind us that the danger of a hard or visible border would really be a target for these mindless yobs who want to haul us all back to our recent dark past," the paper said in an editorial.
"This danger must galvanise all our political leaders to do everything they possibly can to avoid such a possibility."
German Foreign Minister Heiko Maas, speaking in Brussels, cited the Derry bombing as a warning sign that Brexit must not disrupt the Northern Ireland peace process.
"I think it s important that there aren t any decisions that ultimately lead to a hard border between Northern Ireland and Ireland because, as we have seen at the weekend, nerves are shattered there as well," he said.
Irish Times security and crime editor Conor Lally said police officers and politicians had been warning Brexit could become "a rallying call for dissident republicans".
Police received a telephone tip-off that the device had been planted and an evacuation was nearing completion when the explosion occurred.
There were no casualties.
Two men in their 20s were arrested in the early hours of Sunday morning. Later in the day two other men, aged 34 and 42, were also arrested in the city.
All remain in custody, a PSNI spokesman told AFP.
The event has been condemned by political parties across the traditional republican and unionist divide in the province, as well as by leaders in the Republic of Ireland.
Prime Minister Leo Varadkar called it "an appalling, reckless and cynical act of terror".