A dissident republican group calling itself the 'New IRA' has admitted responsibility for the murder of journalist Lyra McKee in Derry.
Ms McKee, 29, died as a result of injuries sustained when she was shot on the Creggan estate last Thursday night, 18 April.
Her murder has been widely condemned and numerous tributes have been paid to her work as a journalist.
The PSNI said this morning that a 57-year-old woman was arrested in connection with Ms McKee's murder.
The woman has been released without charge and PSNI detectives have continued their appeal for information in their investigation.
Two teenagers arrested over the weekend were also released without charge.
In a statement given to The Irish News using a recognised code word, the dissident group offered "full and sincere apologies" to her family and friends.
The 'New IRA' is an amalgam of armed groups opposed to the peace process and it recently claimed responsibility for parcel bombs sent to London and Glasgow in March.
Police 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 1916 Easter Rising.
The group said: "On Thursday night following an incursion on the Creggan by heavily armed British crown forces which provoked rioting, the IRA deployed our volunteers to engage.
"We have instructed our volunteers to take the utmost care in future when engaging the enemy, and put in place measures to help ensure this.
"In the course of attacking the enemy Lyra McKee was tragically killed while standing beside enemy forces
"The IRA offer our full and sincere apologies to the partner, family and friends of Lyra McKee for her death."
Major Investigation Team detectives have arrested a 57-year-old woman under the Terrorism Act in connection with the murder of Lyra McKee in Creggan in Derry/Londonderry on Thursday, 18th April. She has been taken to Musgrave Serious Crime Suite.— PSNI (@PoliceServiceNI) April 23, 2019
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The dissident group has been linked with four murders, including policeman Ronan Kerr, who was killed by a bomb placed under his car in Omagh in 2011.
The group is also linked to the deaths of prison officers David Black, who was shot as he drove to work at Maghaberry Prison in 2012, and Adrian Ismay, who died in 2016 after a bomb exploded under his van outside his home in east Belfast.
The 'New IRA' is believed to have been formed between 2011 and 2012 following the merger of a number of smaller groups, including the Real IRA - the group behind the 1998 Omagh bomb.
It is strongest in Derry, north and west Belfast, Lurgan in Co Armagh, and pockets of Tyrone, including Strabane.
This year, the group was responsible for a car bomb outside the courthouse in Bishop Street, Derry.
The explosives-laden car was left on the city centre street on a Saturday night in January, and dozens of people, including a group of teenagers, had walked past before it detonated.