DUBLIN — The police in Northern Ireland on Friday blamed militants opposed to British rule there for the killing of a young journalist who was covering a night of violent unrest in Londonderry.

“We are treating this as a terrorist incident, and we have launched a murder inquiry,” Mark Hamilton, an assistant chief constable, said of the fatal shooting of the journalist, Lyra McKee, 29.

A deputy chief constable, Stephen Martin, told reporters that the police were searching for several suspects. “There was certainly more than one person involved,” he said, adding: “This was not done to further any cause. This will have achieved nothing other than to plunge a family into grief.”

[Update: 2 men arrested in killing of journalist Lyra McKee.]

Mr. Hamilton said the police were attributing the violence and the killing to the New Irish Republican Army, a militant republican group formed several years ago from the merger of several splinter groups. It is not affiliated in any way with the Provisional Irish Republican Army, which renounced violence in 2005.