A police hunt is under way in Derry for the dissident republican gunman who killed journalist Lyra McKee in an attack that immediately prompted warnings that political violence must not be allowed to take hold in Northern Ireland again.

As family, friends and supporters gathered on Friday at a vigil to mourn the 29-year-old investigative journalist and activist, police said dissident republicans from a group known as the New IRA were likely to be responsible for the killing and launched a murder investigation.

McKee was shot during rioting in Derry on Thursday while standing near a police Land Rover. The Police Service of Northern Ireland (PSNI) said the shooting was part of an orchestrated attack on its officers and described it as a terrorist incident.

Play Video 1:03 Police release CCTV footage of Lyra McKee in crowd moments before she was shot – video

Police released CCTV showing McKee in the crowd and the man suspected of firing the shots that killed her. Det Supt Jason Murphy said: “The footage also shows the gunman at the corner and an individual picking up something from the ground on the same corner. We are releasing this to encourage anyone with information to make contact with us.”

Q&A What is the New IRA? Show Hide The New IRA is the biggest of the dissident republican groups operating in Northern Ireland. It has been linked with four murders, including the shooting of journalist Lyra McKee in Derry in April 2019. The group is believed to have formed between 2011 and 2012 after the merger of a number of smaller groups, including the Real IRA, which was behind the 1998 Omagh bombing. Its presence is strongest in Derry, north and west Belfast, Lurgan in County Armagh, and pockets of Tyrone, including Strabane. In January 2019 the group was responsible for a car bomb outside the courthouse in Derry. The explosives-laden car was left on Bishop Street on a Saturday night, and scores of people, including a group of teenagers, had walked past before it detonated. The New IRA also claimed responsibility for a number of package bombs posted to targets in London and Glasgow in March 2019.

Amid anxiety over the possibility of tensions escalating, the leaders of Northern Ireland’s six main political parties said they were “united in rejecting those responsible for this heinous crime”. In a rare joint statement, issued exactly 21 years after the Good Friday agreement, they said: “Lyra’s murder was also an attack on all the people of this community, an attack on the peace and democratic processes.” It added: “This is a time for calm heads.”

Facebook Twitter Pinterest Lyra McKee. Photograph: Jess Lowe/AFP/Getty

The Irish prime minister, Leo Varadkar, said: “We cannot allow those who want to propagate violence, fear and hate to drag us back to the past.”

McKee, a published author who in 2016 was named by Forbes magazine as one of the “30 under 30 in media”, had published investigative pieces with BuzzFeed and the Atlantic, and was an editor for California-based news site Mediagazer, a trade publication covering the media industry.

She recently signed a two-book deal with Faber & Faber. Her last tweet on Thursday said: “Derry tonight. Absolute madness.”

In scenes of high emotion at Friday’s vigil, her partner, Sara Canning, said: “Our hopes and dreams and all of her amazing potential were snuffed out by this single barbaric act. Victims and the 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.

“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.”

Play Video 1:44 Lyra McKee's partner and politicians pay tribute to killed journalist - video

One of those who paid tribute to McKee at the vigil, which was attended by several hundred people, was the Booker prize winner Anna Burns. She revealed afterwards that she had been due to have dinner with McKee in Belfast that night.

The attack is not the first by the New IRA, which has been active in Derry, Northern Ireland’s second city. The group emerged in 2012 via a merger of several groups opposed to the peace process, including the Real IRA.

It has been linked to the murder of two prison officers and several other attacks. It detonated a large car bomb outside the city’s courthouse in January. Recently, a horizontal mortar tube and command wire were discovered near Castlewellan in County Down.

The PSNI temporary deputy chief constable, Stephen Martin, appealed to families of members of violent dissident organisation to urge them to “step away” from the violence. “There are people in this city who will know the people they love are involved in organisations like the New IRA. I would urge those people to have conversations in their home, in their family space, in Lyra’s memory,” he said.

The Belfast-born investigative journalist was rushed to hospital in the back of a police Land Rover after the shooting, but died from her injuries.

Play Video 2:18 Petrol bombs thrown at police in Derry as journalist killed - video

Trouble flared when police entered the Creggan estate, traditionally a republican stronghold, at around 9pm to search for guns and explosives they believed were being stored for planned attacks over Easter, when republicans mark the anniversary of the 1916 Easter Rising.

A crowd gathered. Up to 50 petrol bombs were thrown and two vehicles were set on fire, police said. By 11pm around 100 people, including journalists, had gathered on the streets when shots were fired.

Leona O’Neill, a Derry-based journalist, was standing beside McKee when McKee fell beside a police Land Rover. “I called an ambulance for her, but police put her in the back of their vehicle and rushed her to hospital where she died. Just 29 years old. Sick to my stomach tonight,” she tweeted.

Local resident Emmet Doyle said in a Facebook post that children were among those gathered in Fanad Drive when the shooting happened. “A masked figure stopped at the bottom of the road and fired shots up towards the Land Rovers. We all turned and ran. I stopped beside the Land Rover nearer the top of the street and a girl beside me dropped to the ground.”

Mobile phone footage obtained by RTÉ News suggested there were at least two gunshots. The footage also showed two masked men appearing to pick up empty bullet casings.

McKee’s death had been met with “global condemnation, horror and revulsion”, said Martin. “The full and total responsibility for Lyra McKee’s death lies with the organisation that sent someone out with a gun,” he said. He confirmed police were looking for multiple suspects.

Saoradh, a fringe political party that reflects New IRA thinking, said the blame for events of Thursday evening lay “squarely at the feet of the British crown forces”. It added: “A republican volunteer attempted to defend people from the PSNI/RUC [Royal Ulster Constabulary]. Tragically, a young journalist covering the events, Lyra McKee, was killed accidentally.”

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Theresa May called the killing “shocking and truly senseless”, adding: “She was a journalist who died doing her job with great courage.”

The former US president Bill Clinton, who helped broker the Good Friday agreement, said he was heartbroken by McKee’s murder: “The challenges in NI today are real – but we cannot let go of the last 21 years of hard-won peace and progress. This tragedy is a reminder of how much everyone has to lose if we do.”