The New IRA has admitted carrying out the murder of journalist Lyra McKee, according to the Irish News.
In a statement given to the paper the group offered "full and sincere apologies" to her family and friends.
Ms McKee, 29, was shot in the head on Thursday night while observing rioting in Londonderry's Creggan estate.
A 57-year-old woman arrested on Tuesday in connection with Ms McKee's death has been released unconditionally.
Police say there has been a "massive response" to her killing and have urged more members of the public to come forward.
The statement from the New IRA comes after the hard-left republican political party Saoradh - which has the support of the New IRA - had previously sought to justify the use of violence on Thursday.
In the House of Common on Tuesday, Northern Ireland Secretary Karen Bradley said there was nothing that could justify this "murderous act".
"To those responsible for this act of terrorism, we say: 'We have heard your excuses and your hollow apologies. No-one buys it. This was no accident'," she added.
Mrs Bradley also said she would hold discussions with the leaders of Northern Ireland's political parties later this week.
She will be among political leaders who will attend Ms McKee's funeral on Wednesday at St Anne's Cathedral in Belfast.
Irish President Michael D Higgins and Taoiseach (Irish PM) Leo Varadkar will also attend.
They will be joined by Irish Foreign Minister Simon Coveney, who is already in Northern Ireland meeting some of the Stormont parties after calls on Monday for urgent talks to be convened to restore power-sharing.
Tánaiste @simoncoveney alongside the Mayor of Derry, Councillor John Boyle, signing the book of condolences for Lyra McKee at the Guildhall in Derry this evening. The President, Taoiseach and Tánaiste will attend Lyra McKee’s funeral in Belfast tomorrow. pic.twitter.com/bmddrkeGY6— Irish Foreign Ministry (@dfatirl) April 23, 2019
A vigil was held in Dublin city centre on Tuesday evening in memory of Ms McKee, which was organised by the National Union of Journalists.
Ms McKee's partner, Sara Canning, said the service on Wednesday would be a "celebration of her life".
Ms McKee was standing near a police 4x4 vehicle when she was shot after a masked gunman fired towards police and onlookers.
A protest by friends of Ms McKee took place on Monday outside the office of Saoradh.
A number of women smeared red paint in handprints on republican slogans outside the office.
Police were present but did not make any immediate arrests.
Police said the public response to the killing had been "massive".
Det Supt Jason Murphy said there had been a "palpable change" in community sentiment in support of their investigation, in terms of off-the-record intelligence.
He has urged members of the public to "come forward and have a conversation with me".
It is understood that police and the Public Prosecution Service have discussed what measures could be available to protect witnesses fearful of giving evidence at trial.
The New IRA is believed to have been formed between 2011 and 2012.
It followed the merger of a number of smaller groups, including the Real IRA, which itself was born out of a split in the mainstream Provisional IRA (PIRA) in October 1997 over Sinn Fein's embrace of the peace process.
The New IRA has been linked with four murders.
Ms McKee's killing came 21 years after the Good Friday peace agreement was signed in Northern Ireland.
The 1998 peace deal marked the end in the region of decades of violent conflict - known as the Troubles - involving republicans and loyalists during which about 3,600 people are estimated to have died.
The Good Friday Agreement was the result of intense negotiations involving the UK and Irish governments and Northern Ireland's political parties.