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.