Two men have been arrested under the Terrorism Act in connection with the murder of journalist Lyra McKee.
Ms McKee, 29, was hit by a bullet and killed during rioting in the Creggan area of Londonderry on Thursday.
The men, aged 18 and 19, were detained under anti-terrorism legislation and have been taken to a police station in Belfast for questioning, the Police Service of Northern Ireland (PSNI) said.
An outpouring of tributes to the 29-year-old was led by her partner, Sara Canning, who said Ms McKee's "amazing potential was snuffed out by this single barbaric act".
Detectives hunting the gunman have released footage of the shooting in the hope the community can help trace her killers.
The published author, from Belfast, was shot in the head by suspects who police believe were dissident republicans linked to the New IRA, as they clashed with police on the Creggan estate in Londonderry.
Mobile phone footage released by police on Friday appears to show the masked shooter fire a handgun towards police and onlookers including Ms McKee.
The published author's partner Ms Canning said it was a senseless murder.
"Our hopes and dreams and all of her amazing potential was snuffed out by this single barbaric act," she said.
Ms Canning said it has left so many friends without their confidante.
"Victims and LGBTQI community are left without a tireless advocate and activist and it has left me without the love of my life, the woman I was planning to grow old with," she added.
"This cannot stand, Lyra's death must not be in vain because her life was a shining light in everyone else's life and her legacy will live on and the life that she has left behind."
In CCTV footage also released by Police Service Northern Ireland, Ms McKee can be seen standing with a crowd of people beside a police vehicle moments before she was shot. She raised her mobile phone into the air, apparently to take a photo of the confrontations.
A third clip shows the masked shooter head-on as he steps from behind a wall then points a handgun towards police and bystanders.
Detectives released the footage, which also appears to show an accomplice picking up something from the ground where the gunman was stood, to encourage anyone with information to make contact.
Police have blamed the anti-peace process New IRA for the killing after handgun shots aimed at their officers were fired indiscriminately in the Creggan estate.
Detectives believe the violence was orchestrated in response to an earlier search by officers aimed at averting imminent trouble associated with this week's anniversary of the Easter Rising.
They said more than one person was involved in the murder.
They have already received a large number of calls and information from the public and have a special portal where video and pictures can be uploaded.
Prime Minister Theresa May said the killing was "shocking and senseless".
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 Londonderry to live with "love of her life" Sara.
She was 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.
"A bright light has been quenched and that plunges all of us into darkness."
Causes close to her heart included helping homeless people, preventing suicide and supporting LGBT rights in the most restrictive regime in the UK.
Catholic priest Father Joseph Gormley comforted Ms McKee's family in hospital and accused the killers of forcing their viewpoint on others using the barrel of a gun.
He asked: "Have you no sense of humanity or dignity about yourself?"
Deputy chief constable Stephen Martin criticised those behind earlier violent scenes during which more than 50 petrol bombs were thrown and two cars burned in the Creggan.
"Regretfully, people such as I described earlier who are completely out of step, goaded and orchestrated young people to engage in disorder," he said.
"The police didn't react to that disorder. We didn't respond with any use of force, we absorbed it.
"We were there to do search activity. We did not want in any way to make the situation worse."
He defended the decision to launch an operation earlier on Thursday aimed at thwarting dissident plans for "imminent" violence.
In the past, trouble has coincided with dissident republican commemoration of the battle for Irish independence every Easter.
Anti-peace process sentiment in Northern Ireland's second city has been demonstrated in recent attempts to bomb the courthouse and the calling off of a community youth event after police were invited.
Mr Martin condemned those whose sole purpose in life was to try to attack his officers and destroy the peace.
"Today is Good Friday and it's a cruel twist in our history that 21 years ago the majority of people in Northern Ireland signed up to the Belfast/Good Friday Agreement yet here we are today mourning the loss of a talented young woman, a young journalist who was also a daughter, a sister and a partner.
"This is a dark day."
Assistant chief constable Mark Hamilton said it was a calculated and callous act to bring a firearm into a residential area.
"Bullets stop somewhere, and on this occasion they stopped fatally."
A vigil was held in the Creggan in Ms McKee's memory, organised by local residents who said they felt sad and angry.