Detectives investigating the murder of Northern Irish journalist Lyra McKee have arrested a 57-year-old woman under the Terrorism Act in connection with the incident, police said Tuesday.

The arrest came a day after police released two men without charge and hours after the armed republican group known as the New IRA told The Irish News newspaper that one of its members shot McKee in the city of Londonderry/Derry on Thursday.

McKee was standing near police officers as dissident republicans clashed with police in the city's Creggan district.

"In the course of attacking the enemy, Lyra McKee was tragically killed while standing beside enemy forces," the New IRA said.

"The IRA offer our full and sincere apologies to the partner, family and friends of Lyra McKee for her death," it added.

What is the New IRA?

The New IRA, which formed in 2012, accused police of provoking rioting that preceded the gun attack in which McKee died.

The group rejects the Good Friday agreement of 1998, which brought to an end three decades of sectarian conflict in Northern Ireland that claimed more than 3,600 lives.

The admission and apology came after far-left republican political group Saoradh, which is associated with the New IRA, called for the organization to apologize.

Saoradh canceled a dissident republican Easter march on Monday "as a mark of respect."

'Blood on their hands'

However, Saoradh has come under heavy criticism for its perceived role in stoking sectarian hatred. 

Friends of the journalist held a protest at Saoradh headquarters on Monday, using a pot of red paint to place handprints on the walls of the building.

"We have used red paint because they have blood on their hands for what has happened," said protester Sinead Quinn, a friend of McKee.

"They have encouraged it, they have molded 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."

Read more: The Irish border — what you need to know

McKee's killing followed a rise in incidents being viewed by police as terror-related, with police warning that "a new breed of terrorist" is emerging from the ranks of paramilitary organizations. This year alone, the New IRA has claimed responsibility for a car bomb in Derry in January and a spate of letter bombs that were sent to British targets in March.

The unrest came ahead of the Easter weekend when republicans, who want Northern Ireland to become part of a united Republic of Ireland, traditionally mark the anniversary of the 1916 Easter Rising against British rule.

Commentators have speculated that Brexit, which has raised the prospect of a hard border between the Republic and Northern Ireland, could be exploited by paramilitaries to revive their causes.

amp, rc/sms (dpa, Reuters, AFP, AP)