Dissident republican group the New IRA was probably responsible for shooting dead Lyra McKee, 29, police have said.
The reporter was shot in the head during a “terrorist incident” amid disturbances in Londonderry on Thursday evening and died later in hospital.
Police Service of Northern Ireland (PSNI) 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 the 1916 Dublin uprising that led to Irish independence.
In a tweet Ms Sturgeon said: “If you read nothing else today, read this letter Lyra McKee wrote to her 14 year old self. It is heartbreaking but life affirming...and leaves me wishing I’d had the chance to meet this brave young woman. RIP Lyra.”
Prime Minister Theresa May said: “The death of Lyra McKee in last night’s suspected terrorist incident in Londonderry is shocking and truly senseless.
“My deepest condolences go to her family, friends and colleagues.
“She was a journalist who died doing her job with great courage.”
Irish Taoiseach Leo Vardakar said his government condemned the violence in the “strongest possible terms”.
And he said: “We cannot allow those who want to propagate violence, fear and hate to drag us back to the past.”
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 added: “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 and can only be designed to hurt and kill people.
“Bullets stop somewhere, and on this occasion they stopped fatally.”
The New IRA is an amalgam of a series of armed groups opposed to the peace process.
It claimed responsibility for a number of parcel bombs sent to London and Glasgow recently.
The threat posed to police in Northern Ireland is high.
It is understood Ms McKee had recently moved to Derry to live with her partner.
She worked as an editor for California-based news site Mediagazer, a trade publication covering the media industry.
In 2016, Forbes Magazine named her one of their 30 under 30 in media. She had been working on a new book which had been due to be published in 2020.
Michelle Stanistreet, National Union of Journalists (NUJ) general secretary, said Ms McKee was one of the most promising journalists in Northern Ireland.
She said: “A young, vibrant life has been destroyed in a senseless act of violence.”
She added: “A bright light has been quenched and that plunges all of us in to darkness.”
Mr Hamilton confirmed she was wounded and taken away in a police Land Rover to Altnagelvin Hospital but died there.
He said she was a “perfectly innocent” bystander with legitimate reason for being there but had not been “actively working” as a journalist on Thursday evening.
“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 offered his “deepest sympathies” to Ms McKee’s family.
He added: “But not only is it a murder of a young woman, it is an attack again on the people of this city.”
A car bomb left outside a courthouse in the city exploded in January. The New IRA were blamed.
Mr Hamilton said: “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.”
He appealed to people with influence to use it to ensure a quiet weekend and future in the 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.”
Local resident George McGowan said they were a proud, courageous and peaceful people as he announced a vigil on Friday afternoon.
He said: “The events of last night have left us all sad, numb, frightened, bewildered, heartbroken, angry, ashamed, guilty and resolute.
“This behaviour is not in our name; we have all been wounded by these actions.
“A young life is lost and we are left feeling vulnerable and ashamed.”
He added: “Come stand with us and send a clear message that this community will not allow anyone to pull us back to the past and that this community will always stand together.”
Creggan parish Catholic priest Fr Joseph Gormley said he saw the young woman lying in hospital with her life cruelly taken from her with her broken-hearted family at her bedside.
He paid tribute to the family’s dignity and accused dissidents of forcing their viewpoint on others using the barrel of a gun.
“Have you no sense of humanity or dignity about yourself?”
Northern Ireland Secretary Karen Bradley said she was “deeply shocked and saddened” to hear of the death.
“My thoughts and condolences are with her family at this time. Those responsible for last night’s violence have nothing to offer anyone in Northern Ireland.
“Their intolerable actions are rejected by the overwhelming majority of people who want to build a peaceful and more prosperous future for everyone in Northern Ireland.”
United States House of Representatives Speaker Nancy Pelosi also paid tribute to Ms McKee during a visit to Stormont.
A minute’s silence was observed during an event she attended in the Long Gallery in respect of Ms McKee during a speech by Northern Ireland Assembly speaker Robin Newton.
During her speech, Ms Pelosi said: “Sadly, Mr Speaker, we join you on the sadness of the tragedy that happened last night, we extend our condolences to the family, and your moment of silence at this time, on Good Friday is especially poignant,” she said.
“You not only had a moment of silence, you presented a sense of resolve that justice would be done and the memory of that young woman would be a lesson for all of us.”