Two men have been arrested in connection with the killing of Northern Irish journalist Lyra McKee during a riot in Londonderry on Thursday, police said, as politicians in the divided UK region united to condemn the attack.
McKee, 29, an award-winning reporter who was writing a book on the disappearance of young people during decades of violence in Northern Ireland, was shot dead as she watched Irish nationalist youths attack the security forces following police raids.
"Major Investigation Team detectives have arrested two men, aged 18 and 19 under the Terrorism Act, in connection with the murder," the Police Service of Northern Ireland said.
Northern Ireland's political parties, which are broadly split between Irish nationalists aspiring to unite the region with the Republic of Ireland and unionists who want it to remain British, united to condemn the attack.
In a joint statement, six parties said they were "united in rejecting those responsible for this heinous crime".
Hundreds gathered in cities across Northern Ireland on Friday to hold vigils for McKee, who was also known as an activist for lesbian and gay rights.
Politicians around the world condemned the attack, with former US President Bill Clinton saying he was "heartbroken".
"We cannot let go of the last 21 years of hard-won peace and progress," Mr Clinton, a key player in the 1998 Good Friday Agreement peace accord, tweeted.
The 1998 deal largely ended three decades of violence in the region, in which over 3,600 died, but several smaller militant groups remain active and launch occasional attacks.
Police said they believed the shooting was likely carried out by the New IRA, a group of "dissident" militants opposed to the Good Friday deal. Police blamed the group for planting a car bomb outside a courthouse in Londonderry, also known as Derry, in January.
Politicians in Northern Ireland said that the UK's plans to leave the European Union could also undermine the peace deal. When the UK is no longer a part of the European bloc, there will be some form of border between the Republic of Ireland – an EU member – and Northern Ireland – part of the United Kingdom.
Part of the agreement that ended the Troubles was the softening of the border between the two EU members to allow unfettered movement.
Officials said that any return of restrictive infrastructure along the Irish border would become targets for militants.
Michel Barnier, the EU's chief Brexit negotiator, said the peace accord had to be preserved.
"The tragic murder of Lyra McKee is a reminder of how fragile peace still is in Northern Ireland," he said.
Police said Thursday's rioting began after a raid aimed at preventing attacks during Easter weekend.
Saoradh, a political party with links to dissident militants, said on Friday that McKee was killed accidentally by a "Republican volunteer" who was “attempting to defend people from the PSNI [Police Service of Northern Ireland],” and blamed the authorities for her death.
McKee was watching with a crowd of bystanders as local youths attacked police with petrol bombs and set cars on fire, video footage showed. Police said McKee was hit when a gunman opened fire in the direction of police.
British Prime Minister Theresa May on Friday described the attack as "shocking and truly senseless".