A small bomb in a backpack damaged the entrance of the City of Culture office in Londonderry, Northern Ireland’s No. 2 city, on Wednesday night. An Irish Republican Army splinter group using a recognized code word claimed responsibility.

The city’s senior policeman, Chief Superintendent Stephen Martin, says the dissidents are too reliant on amateur timing devices and crude homemade bombs. He says Wednesday’s blast an hour after the warning was strong enough to kill a passerby.

Martin says it’s “only a matter of time before a mistake does happen and these people take a life.”