MORE than one person was involved in an orchestrated bid to murder​ police that left one of Northern Ireland's most promising young journalists dead, senior officers have said.

Published author Lyra McKee, 29, was an innocent bystander shot in the head by dissident republicans during serious disturbances in Derry on Thursday evening, they added.

Detectives from the Police Service of Northern Ireland (PSNI) blamed the anti-peace process New IRA and said the intention was to kill its officers.

Deputy chief constable Stephen Martin said: "They were not on their own. There was more than one person who was involved in this last night."

Mobile phone footage taken by a bystander appeared to show a masked gunman crouching down on the street in the Creggan estate and firing with a handgun.

McKee was standing near a police vehicle and was fatally wounded. She was taken to hospital by officers but later died.

READ MORE: New IRA suspected of being behind Derry car bomb attack

Assistant Chief Constable Mark Hamilton said: "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."

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 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.

McKee's partner Sara 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 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."

Deputy chief constable of the Police Service of Northern Ireland (PSNI) Stephen Martin said the hearts of people in the city were breaking.

He added: "They have been grossly disappointed with the image of their city across the world today."

He defended the decision to launch an operation earlier on Thursday aimed at thwarting dissident plans for "imminent" violence.

Violence broke out soon afterwards.

The senior officer said: "Police were in that estate last night carrying out lawful activity, trying to prevent future imminent violence.

"The full and total responsibility for Lyra McKee's death lies with the organisation that sent someone out with a gun."

He said he has viewed intelligence and convinced himself of the urgency and need for the operation.

Hamilton offered his "deepest sympathies" to McKee's family.

He said: "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.

"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."

The New IRA, an amalgamation of dissident republican factions in Derry, has been blamed for a bombing in Bishop Street, Derry, in January.