The Northern Ireland Police Service say they have arrested a woman under the Terrorism Act in connection with the slaying of journalist Lyra McKee.
The arrest of the 57-year-old under the Terrorism Act came as an Irish Republican Army splinter group admitted that one of its "volunteers" killed journalist McKee, who was shot dead last week while reporting on rioting in Londonderry.
In a statement given to The Irish News using a recognised code word, the New IRA 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."
The IRA and most other militant groups have disarmed since Northern Ireland's 1998 peace accord.
The New IRA has been formed from splinter groups opposed to the peace process.
Authorities believe one person pulled the trigger during the chaotic rioting but had organisational support.
The use of a firearm apparently aimed at police marks a dangerous escalation in sporadic violence that continues to plague Northern Ireland 21 years after the Good Friday peace agreement was signed. The New IRA group rejects the peace agreement.
The riot followed a pattern familiar to those who lived through the worst years of violence in Northern Ireland.
Police arrived in the city's Creggan neighbourhood to search for weapons and dissidents. They were barraged with gasoline bombs and other flying objects before someone wearing a black mask appeared, fired some shots and fled.
No police were struck by the bullets, but McKee — who had been trying to film the riot on her phone — was hit. The journalist was rushed to a nearby hospital in a police car but died.
Detective Superintendent Jason Murphy said that since the 29-year-old's death more than 140 people had contacted police with information.
Friends of the journalist protested by defacing an office belonging to a dissident republican group by putting red handprints on the walls of its headquarters.
A number of Miss McKee's friends walked to Junior McDaid House, where they used a pot of red paint to place handprints on the side of the office walls.
A group of some six men, understood to be members of republican group Saoradh, who are associated with the New IRA, stood outside the building during the protest.
PSNI officers were also present and later asked for the names of those involved in the incident.
McKee's killing has been widely condemned across the political spectrum.
Lyra's friend Sinead Quinn, who took part in the protest, said: "We have used red paint because they have blood on their hands for what has happened.
They have encouraged it, they have moulded these young people into what they are and they are standing behind them handing them guns. Sinead Quinn
"They have encouraged it, they have moulded these young people into what they are and they are standing behind them handing them guns.
"They need to take responsibility today for what has happened.
"They have shirked it so far by saying it was an accidental shooting. You don't shoot accidentally."
She added: "When you put a gun into someone's hand and they shoot it, that's murder.
"Lyra deserves more and I am so glad there are so many people here today to see and watch these men looking at us.
"They are not a representation of republican people in this town.
"Those people don't represent (republicanism). Nobody can advocate shooting into a crowd of people and shooting a 29-year-old woman dead.
"People have been afraid to stand up to people like this, we are not afraid."
The group of friends have pledged to do more in Lyra's memory.
"Lyra's McKee's name will never be forgotten in this town," Quinn added. "We have to do it for her."
- AP, Telegraph Media Group