LONDONDERRY (Reuters) - Two men have been arrested in Northern Ireland over the killing of a journalist during a riot, police said, with politicians calling for calm after the shooting added to concerns about the stability of the region’s 21-year-old peace deal.

Lyra McKee, 29, an award-winning journalist who was writing a book on the disappearance of young people during decades of violence in Northern Ireland, was shot dead on Thursday as she watched Irish nationalist youths attack police following a raid.

Her death, which follows a large car bomb in the city in January, raised fears that small marginalized militant groups are trying to exploit political tensions caused by Britain’s decision to leave the European Union.

Northern Ireland’s political parties, which are broadly split between Irish nationalists aspiring to unite the British region with Ireland and unionists who want it to remain British, issued a joint call for calm.

“Lyra’s murder was an attack on all the people of this community, an attack on the peace and on the democratic processes,” leaders of the main six parties said in a statement. “This is a time for calm heads.”

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 officers.

The Police Service of Northern Ireland said on Saturday that two men, aged 18 and 19, had been arrested under the Terrorism Act in connection with the murder.

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Police said they believed the shooting was likely carried out by the small New IRA group of Irish nationalist militants opposed to the Good Friday peace deal. The group was blamed by police for planting the car bomb outside a courthouse in Londonderry in January.

It is one of a number of small groups who remain active and oppose the 1998 deal, which largely ended three decades of violence in the region in which over 3,600 died.

“FRAGILE PEACE”

Politicians around the world condemned the shooting, with former U.S. President Bill Clinton, a key player in the 1998 Good Friday Agreement peace accord, saying he was “heartbroken.”

“We cannot let go of the last 21 years of hard-won peace and progress,” Clinton said on Twitter.

Politicians in Northern Ireland have warned that Britain’s plans to leave the European Union could undermine the peace deal and that any return of restrictive infrastructure along the Irish-Northern Irish border would become targets for militants.

Michel Barnier, the European Union’s chief Brexit negotiator, who has insisted the maintenance of a completely open border, on Saturday said the killing was “a reminder of how fragile peace still is in Northern Ireland.”

Hundreds had 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.

In a rare show of unity, the leader of the pro-British Democratic Unionist Party Arlene Foster joined the head of rival Sinn Fein in a heavily Irish nationalist area close to the scene of the shooting to condemn McKee’s killing.