Dissident republican group the New IRA was probably responsible for shooting dead a journalist in Northern Ireland, police said.
Lyra McKee, 29, was shot in what police are treating as a "terrorist incident", amid disturbances in Londonderry on Thursday evening, and died later in hospital.
Police Service of Northern Ireland officers were carrying out a search operation in the Creggan area of Derry aimed at disrupting dissident republicans ahead of this weekend's commemoration of Irish independence, when a situation developed during which more than 50 petrol bombs were thrown at officers and two cars were hijacked and set on fire.
During the unrest a gunman fired a number of shots at police, Assistant Chief Constable Mark Hamilton said.
He said: "Lyra McKee, 29 years old, was wounded. She was taken away in a police Land Rover to Altnagelvin Hospital but unfortunately she has died there.
"We have now launched a murder inquiry here in the city.
"We believe this to be a terrorist act, we believe it has been carried out by violent dissident republicans, our assessment at this time is that the New IRA are most likely to be the ones behind this and that forms our primary line of inquiry.
"This is a horrendous act, it is unnecessary, it is uncalled for, it is totally unjustified."
Mr Hamilton called the violence "an attack on the people of this city".
He added: "But not only is it a murder of a young woman, it is an attack again on the people of this city.
"I stood here in January and we talked about the bomb and the act of violence against this city, and yet again we see another act of violence in this city which has had horrendous consequences and which will affect people for many, many years.
ACC Mark Hamilton says ‘our thoughts are very much with the family & friends of Lyra McKee’. He says her murder ‘demonstrates all too starkly when terrorists bring violence & guns into the community members of the public are placed in severe danger’. pic.twitter.com/26TJZMkLGD— PSNI (@PoliceServiceNI)April 19, 2019
"I would appeal to people, particularly this Easter weekend, to stay calm, I would appeal to people who are intent on violence to draw back, I would appeal to people with influence to use your influence and make sure this is peaceful weekend. Not only this weekend but going forward in this city.
"These acts of violence are bringing nothing to this city, all they are doing is bringing misery to one family, but also particularly to this city and also to our broader province."
It is believed Ms McKee was not on duty working when she was shot.
Mr Hamilton said it is understood the Belfast native had recently moved to Londonderry to live with her partner, and had been working as an editor for California-based news site Mediagazer, a trade publication covering the media industry.
She had also been named as one of Forbes Magazine's 30 under 30 in media, in 2016.
The unrest comes ahead of the Easter weekend where republicans mark the anniversary of the 1916 Easter Rising, a time when dissidents are traditionally active.
Politicians have widely condemned the killing.
Sinn Fein deputy leader Michelle O'Neill said the killing in Derry was a "senseless loss of life".
She said: "I am shocked and saddened at the tragic news that a young woman has been shot dead by so-called dissidents in the Creggan estate tonight.
"The murder of this young woman is a human tragedy for her family, but it is also an attack on all the people of this community, an attack on our peace process and an attack on the Good Friday Agreement.
"I unreservedly condemn those responsible for killing this young woman.
"We will remain resolute in our opposition to the pointless actions of these people who care nothing for the people of Derry."
Democratic Unionist Party leader Arlene Foster tweeted: "Heartbreaking news. A senseless act. A family has been torn apart.
"Those who brought guns onto our streets in the 70s, 80s & 90s were wrong. It is equally wrong in 2019.
"No one wants to go back. My thoughts are also with the brave officers who stood in defence of their community."
Mr Hamilton has also appealed for information and witnesses to come forward.
"If people have information, I would appeal to you to bring it forward, you can ring Crimestoppers, you can ring 101 and ask for police at Strand Road," he said.
"We need people who saw this happen to come forward. We also will be putting out a portal for people to upload videos they may have taken, I know a lot of this is already circulating on social media.
"I would ask people to be sensitive to the family, someone has lost their life and the last thing that family needs is to see that played over and over again.
"I would have people who have videos show sensitivity, either not show them or share them with police so we can use them in our murder inquiry to try and track down the people who have committed this horrendous crime."
Responding to a question about the current level of threat from dissident republican terrorists and whether the incident was orchestrated, Mr Hamilton said he believes the incident was orchestrated but "clearly reckless".
"It was orchestrated to the point where they just want to have violence and want to primarily, on this occasion, to attack police. But the bringing of a firearm out and firing it down a street in a residential area where they knew lots of people were standing about is a calculated and callous act."