Police in Northern Ireland are still investigating the death of 29-year-old journalist Lyra McKee, who was shot Thursday during a police riot in Derry, also known as Londonderry.

The Police Service of Northern Ireland arrested a 57-year-old woman on Tuesday in connection to the McKee’s killing, but police later said she was released “unconditionally.”

This came the same day a dissident group known as the New Irish Republican Army (IRA) took responsibility for the shooting, though it apologized for her death in a statement, indicating that McKee was not the intended target.

“In the course of attacking the enemy Lyra McKee was tragically killed while standing beside enemy forces,” the statement, which was given to the Irish News, said. “The IRA offer our full and sincere apologies to the partner, family and friends of Lyra McKee for her death.”

The statement from the new IRA accused “heavily armed British crown forces” for “provoking the rioting” in Creggan estate, a largely Catholic housing complex in Derry.

McKee, a journalist who has documented Northern Ireland’s ongoing peace process, was killed in the riots that erupted after police raided Creggan in search of explosives and weapons they believed were about to used by New IRA dissidents in attacks. Rioters threw Molotov cocktails at police, and two cars were hijacked and set on fire.

McKee had been reporting on the confrontation when she struck in the head by a bullet when someone fired at police officers. McKee died of her injuries at the hospital.

Two teenagers were taken into custody shortly after McKee’s death, but both were later released. Now, the 57-year-old woman has also been released.

The new IRA’s violence revives concerns about tensions in Northern Ireland

The New IRA is a nationalist paramilitary group that formed around 2012. Though it often refers to itself as “the IRA,” the majority of Provisional IRA members disarmed as part of the Good Friday Agreement — the landmark peace deal, signed in 1998, that ended decades of conflict in Northern Ireland known as the Troubles.

That conflict, which claimed nearly 3,600 lives, was largely fought by paramilitary groups representing nationalists, a largely Catholic population who sought a united Ireland, and paramilitary groups representing unionists, who sought to preserve British rule. (The IRA were republicans who advanced the nationalist cause through violence.) Law enforcement and the British military were also to blame for some of the violence.

The new IRA is made up of fragments of the Provisional IRA who didn’t buy into the peace process and still want to kick out the British and unite Ireland by force.

This New IRA doesn’t have broad support among the population it claims to represent, and there’s been widespread pushback against the violence in Derry last Thursday.

The Police Service of Northern Ireland said it had received more than 140 tips about McKee’s murder, and supporters of McKee protested by putting blood-red handprints on the wall of the headquarters of Saoradh, a political group that’s tied to the New IRA.

A number of Lyra McKee's friends walked to Junior McDaid House in Derry, where they used a pot of red paint to place handprints on the side of the office walls https://t.co/BZyZiY6Lpd — RTÉ News (@rtenews) April 22, 2019

Six major political parties in Northern Ireland — including Sinn Féin, which has historic ties to the IRA — put out a joint statement condemning McKee’s murder, calling it “an attack on the peace and democratic processes.”

“It was a pointless and futile act to destroy the progress made over the last 20 years, which has the overwhelming support of people everywhere,” the statement read.

But despite the near-universal condemnation of McKee’s killing, her death has heightened anxieties about a return to the Troubles, when the public became entrapped in a cycle of political violence and terror attacks.

Those concerns have been growing in recent months as the New IRA has carried out a series of violent incidents, including a car bombing in Derry in January and a slew of package bombs sent to locations in the United Kingdom, including London and Glasgow, Scotland.

The fact that McKee’s death occurred on the eve of Good Friday in 2019, 21 years after the peace deal was signed, added to those anxieties.

Of course, all of this is taking place against the backdrop of Brexit, with the question of the future of the border between Northern Ireland and the Republic of Ireland threatening to reignite tensions in the region.

What does Brexit have to do with this?

The 30-year conflict known as the Troubles officially ended in 1998 with the signing of the Good Friday Agreement, and now Brexit is threatening to unravel the fragile peace that agreement helped create.

At issue is the 310-mile Irish border, the land boundary that separates Northern Ireland — which is part of the United Kingdom — from the Republic of Ireland, an independent country that is also a member of the European Union.

That border was heavily militarized during the Troubles, serving as both a symbol of the strife and a very real target for attacks by nationalist paramilitary groups. A fundamental pillar of the Good Friday Agreement that ended the conflict involved greater cooperation between Northern Ireland and Ireland. That meant softening the border between the two.

The European Union strengthened this truce, as its rules on trade and movement created the conditions for closer ties between the UK and Ireland. The watchtowers came down, the checkpoints disappeared. Now, the boundary is all but invisible.

Whether it will stay that way is one of the most critical questions of the Brexit negotiations between the UK and the EU. Once the two split up, the Irish border will become the land border between the UK and Europe. If a hard border is established, many fear it could inflame still-simmering divisions by becoming a target for dissident groups — like the New IRA — once again.