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.”
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.
Many people in Northern Ireland, primarily Roman Catholics, consider themselves republicans, meaning that they want the region to break away from the United Kingdom and join the Republic of Ireland. But the number who pursue that end through violence is relatively small.
The violence took place in Creggan, a heavily Catholic area of Londonderry — a city that Catholics and republicans generally call Derry — after the police started carrying out searches in the area because of concerns that militant republicans were storing firearms and explosives there in preparation for an attack to commemorate the 1916 rebellion in Dublin known as the Easter Rising.
The searches were followed by a riot in which gasoline bombs were thrown — more than 50, according to the police — and then the attack that killed Ms. McKee. Mr. Hamilton, the assistant chief constable, said she was hit by a gunman firing toward the police.
“We saw unrest last year in the run-up to Easter and this year it’s exactly the same,” said Marisa McGlinchey, an expert on dissident republicans at Coventry University. “We’ve seen an escalation of activity coming up to Easter, and then you have the police responding with searches and arrests, which can lead to street riots.”
The outburst of violence was an ominous reminder of the deadly conflict between militant groups of Catholic republicans and Protestant loyalists, who favor remaining part of Britain, that plagued Northern Ireland until a peace agreement was signed in 1998.
“It’s important that we all stand together now and resist any further escalation of this,” Mr. Hamilton said.
Saoradh (Irish for “Liberation”), a group linked to the New I.R.A., said in a statement that “the blame for last night lies squarely at the feet of the British crown forces, who sought to grab headlines and engineered confrontation with the community.”
The group said that a “republican volunteer” had tried to defend Creggan from an attack by the police, which had prompted the riot. “Tragically a young journalist covering the events, Lyra McKee, was killed accidentally while standing behind armed crown force personnel and armored vehicles,” the statement said.
In her last Twitter post — which was retweeted by Naomi O’Leary, a friend and fellow journalist — Ms. McKee wrote: “Derry tonight. Absolute madness.”
The violence on Thursday came amid the turmoil caused by Britain’s plans to leave the European Union, a bloc that includes the Republic of Ireland. There are concerns that Britain might leave without any agreement defining its relationship to the union, leading to a “hard border” between Ireland and Northern Ireland, with barriers and checkpoints.
Northern Ireland has not had a functioning government for over two years, since the collapse of a power-sharing agreement between its two main parties — one mostly Protestant, the other mainly Catholic.
The Brexit border paradox and the political deadlock in Northern Ireland have, in turn, raised concerns about a resumption of violence, particularly after several packages containing explosives were found last month at the University of Glasgow and three London transport hubs, including Heathrow Airport. The authorities attributed the packages to a splinter group of the Irish Republican Army.
“I don’t have any doubt that Brexit is something that is going to invigorate them and bring them out of the shadows,” said Darach MacDonald, a writer and journalist based in Londonderry.
Despite the outside perception that the 1998 Good Friday peace agreement ended violence in Northern Ireland, police figures for weapons seizures, arrests, assaults and armed attacks remain high, Ms. McGlinchey said, although nothing on the scale of the 1968 to 1998 era of “the Troubles.”
Mr. MacDonald also sees a cultural force at work, a perverse nostalgia for the Troubles, particularly among bored urban youths too young to have experienced them. He says that while the shots that killed McKee may have been fired by a republican militant, most of the young men rioting in the streets were most likely unaffiliated, merely participating in what has become, in many run-down areas of Northern Ireland, a sort of folk ritual.
“It would be something that they would see on news reels,” he said. “A large part of the youth subculture in Derry is now a re-enactment of the past.” He added: “Part of the culture here is a nostalgia for a period when people in the outside world took heed of what happened here. There is an anger about being sidelined, and I suppose that does hanker back to a period when Derry mattered, for all the wrong reasons, but it mattered.”
The apparently accidental victim of the shooting, Ms. McKee, was an investigative journalist from Belfast who was recognized by Forbes magazine in 2016 as one of its “30 Under 30 Europe” for digging “into topics that others don’t care about.”
She had worked as an editor for the website Mediagazer since 2011 and had written for BuzzFeed News and Mosaic Science.
“Letter to My 14-Year-Old Self,” an article she wrote about the abuse and support she received while growing up gay, was widely read online and made into a short film.
Ms. McKee grew up on Cliftonville Road in Belfast, near the area that saw some of the worst violence during the Troubles. Her work often examined the legacies of that era.
She said people had gathered in the streets after a large number of police officers started searching a home in the area.
After violence erupted, the police appealed for calm.
The killing happened the same day that Speaker Nancy Pelosi visited the city to show support for the peace agreement that the United States helped broker two decades ago. Ms. Pelosi said it was vital to keep a “seamless border” after Britain’s withdrawal from the European Union.
Ms. McKee wrote a book, “Angels With Blue Faces,” about the 1981 murder of the Rev. Robert Bradford, a member of the British Parliament from Belfast.
Last year, she signed a two-book deal with the publishing house Faber & Faber. One of those books, “The Lost Boys,” investigates the disappearances of young men in Belfast in the late 1960s and 1970s, and is scheduled for release next year.