The dissident republican group, the New IRA, was most likely responsible for the fatal shooting of a journalist during overnight rioting in the city of Londonderry, police in Northern Ireland said Friday.
The Police Service of Northern Ireland said 29-year-old journalist and author Lyra McKee died after she was shot during rioting in the Creggan area.
Assistant Chief Constable Mark Hamilton said a gunman fired a number of shots at police during the unrest Thursday evening.
"We believe this to be a terrorist act," he said. "We believe it has been carried out by violent dissident republicans."
Hamilton said the force's assessment "is that the New IRA are most likely to be the ones behind this and that forms our primary line of inquiry."
A murder investigation has been launched but there have been no arrests. Hamilton appealed for calm to prevail on Easter weekend.
U.K. Prime Minister Theresa May called the death of McKee "shocking and truly senseless."
"She was a journalist who died doing her job with great courage," May said.
An eyewitness told the BBC that a gunman fired indiscriminately into a crowd during riots on the crowded Creggan housing complex.
The New IRA is a small group of republicans who reject the 1998 Good Friday agreement that marked the Irish Republican Army's embrace of a political solution to the long-running violence known as "The Troubles" that claimed more than 3,700 lives.
The group is also blamed for a Londonderry car bombing that did not cause any injuries in January. It is regarded as the largest of the splinter dissident groups still operating and has been linked to several other killings in the past decade.
There has been an increase in tensions in Northern Ireland in recent months with sporadic violence, much of it focused in Londonderry, also known as Derry.
McKee, the victim of the shooting, rose to prominence in 2014 with a moving blog post — "Letter to my 14 year old self" — describing the struggle of growing up gay in Belfast, the capital of Northern Ireland.
In the post, she described the shame she felt at 14 as she kept the "secret" of her sexual preference from her family and friends and the love she received when she was finally able to reveal it.
McKee had recently signed a contract to write two books.
Hours before her death she tweeted a photo of the riot with the words: "Derry tonight. Absolute madness."
Seamus Dooley, Irish secretary of the National Union of Journalists, said he was shocked by the murder of a journalist "of courage, style and integrity."
He offered sympathy to "her partner, her family and many friends."