(CNN) A 57-year-old woman who was arrested Tuesday in connection with the investigation into the murder of Northern Ireland journalist Lyra McKee was released several hours later, police said.

McKee, 29, was shot in the Creggan area of Londonderry in what police in Northern Ireland described as a "terrorist attack." The prominent freelance journalist had written for publications including The Atlantic and Buzzfeed News.

The city -- which is referred to by Irish nationalists as Derry and British unionists as Londonderry -- is a short drive from the border with the Republic of Ireland.

"A 57-year-old woman arrested this morning by detectives investigating the murder of Lyra McKee has now been released unconditionally," the Police Service of Northern Ireland (PSNI) said in a statement on Twitter on Tuesday evening.

"Detectives from PSNI Major Investigation Team continue to appeal for help from the local community in Creggan or anyone with information."

On Monday, a dissident Republican group purporting to be the New IRA admitted responsibility for the killing of McKee during rioting last week and apologized to her family, according to The Irish News, which published the statement from the group on Monday.

In a statement to The Irish News using what the newspaper described as a "recognized codeword," the group said: "In the course of attacking the enemy, Lyra McKee was tragically killed while standing beside enemy forces."

Lyra McKee

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

According to The Irish News, the purported New IRA statement was signed with the name "T O'Neill."

CNN has not independently verified the authenticity of the statement and has reached out the Police Service of Northern Ireland for comment.

Investigation

Prior to the alleged admission, police had blamed the New IRA for McKee's death.

According to Detective Superintendent Jason Murphy, McKee was killed by shots that were "fired indiscriminately."

"The shots were fired in a residential area at a time when there were large numbers of local people on the street including children," Murphy told media. "The gunman showed no thought for who may have been killed or injured when he fired these shots."

Police secure the area where a journalist was fatally shot amid rioting overnight in the Creggan area of Derry in Northern Ireland on April 19, 2019.

On Saturday, police arrested two teenagers aged 18 and 19 under the Terrorism Act, but both were released without charge the following day.

Murphy appealed again on Monday for information but noted that over 140 people had already come forward.

Images showed vehicles blazing as a crowd threw fireworks and petrol bombs during the night of rioting, one day before Good Friday.

During Easter weekend, republicans seeking to unite British-ruled Northern Ireland with the Republic of Ireland mark the anniversary of the 1916 revolt in Dublin against British forces. The Easter Rising, mounted at a time when all of Ireland was under British rule, is considered among the most important dates in the struggle for Irish independence.

Police had been in the Creggan area searching for "violent dissident republicans" whom they believed were storing firearms and explosives for planned attacks, possibly over the Easter weekend.

Who is the New IRA?

The original IRA was active in the 20th century in the fight for Irish independence from British rule, prior to the creation of the Irish free state. The group widely known as the IRA is actually called the Provisional IRA, a paramilitary Irish nationalist group active during the "Troubles" -- the decades-long sectarian conflict in Northern Ireland in which more than 3,500 people died and which was ended by the Good Friday Agreement of 1998.

Flowers and tributes are left near the spot where Lyra McKee was shot on Thursday night.

The New IRA has been quietly growing in recent years, although it remains small. It's unclear what if any links there are between the Provisional IRA and the New IRA.

In January, a car bombing in central Londonderry was linked to the New IRA. At the time, Assistant Chief Constable Mark Hamilton described it as "small, largely unrepresentative and just determined to drive people back to somewhere they don't want to be."

There are fears that Brexit could fuel a revival of sectarian violence.

Some fear that Britain's departure from the European Union will mean the reintroduction of border posts on the frontier between Northern Ireland, part of the UK, and the Republic of Ireland, which will remain a European Union member. Border infrastructure was often targeted by Irish nationalist paramilitaries during the "Troubles."

This story has been updated to reflect the latest information from police in Northern Ireland.