The police in Northern Ireland said on Friday that the killing of a journalist who was covering a night of violent unrest in Londonderry was committed by a militant opponent of British rule there.
“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.
“At this stage, we believe her murder was carried out by a violent dissident republican,” he said.
“Our assessment at this time would be that the New I.R.A. are most likely to be the ones behind this,” he added, referring to a militant republican group formed several years ago from the merger of several splinter groups.
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.
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.
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.
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 more than 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.
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.”
In her Twitter feed, Ms. O’Neill described people throwing gasoline bombs, bricks and bottles at the police in the Creggan area. Videos showed vehicles on fire.
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.
Sinn Fein, the former political wing of the Irish Republican Army, and the Democratic Unionist Party condemned the killing of Ms. McKee. Sinn Fein called it an “attack on all the community.”
Ms. McKee wrote a book, “Angels With Blue Faces,” about the 1981 murder of 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.
“McKee has that knack of engaging the head and the heart,” Laura Hassan, the editorial director at Faber, said last year when Ms. McKee’s deal was announced. “The fate of these children is deeply affecting, and we’re engaged too with her argument that these missing children tell us something of a whole lost generation .”