BELFAST (Reuters) - A 29-year-old Northern Irish journalist was shot dead during rioting in Londonderry overnight, an attack that shocked the region and police said was likely the work of Irish nationalist militants opposed to the 1998 Good Friday peace deal.

Rioting hit the Irish nationalist Creggan area of the city in Northern Ireland late on Thursday after a raid by police, who said they were trying to prevent militant attacks this weekend. At least 50 petrol bombs were thrown and two cars set on fire.

Journalist Lyra McKee, 29, was shot dead shortly after posting a picture on Twitter of the violence, which she described as “absolute madness.”

“Unfortunately at 11 o’clock last night a gunman appeared and fired a number of shots toward the police and a young woman, Lyra McKee, 29 years old, was wounded” and later died, Police Service of Northern Ireland Assistant Chief Constable Mark Hamilton told journalists.

McKee, who was named Sky News Young Journalist of the Year in 2006, was writing a book on the disappearance of young people during the three decades of sectarian in Northern Ireland that largely ended with the 1998 deal.

She had also written about her struggles growing up gay in the British province.

The death of McKee, described by her publisher Faber as a rising star of investigative journalism, was condemned by both Irish Catholic nationalist and pro-British Protestant unionist politicians.

British Prime Minister Theresa May described the attack as “shocking and truly senseless.”

“This was an act of hate,” Irish Prime Minister Leo Varadkar said in a statement in front of government buildings in Dublin. “This was an attack not just on one citizen, it was an attack on all of us, our nation and our freedoms.”

Police said they were treating the incident as a terrorist attack and had opened a murder inquiry.

The New IRA (Irish Republican Army) group, which has been responsible for several attacks in recent years, was most likely behind the killing, Assistant Chief Constable Hamilton said.

Saoradh, a political party with links to dissident militants who still embrace violence to merge Northern Ireland with Ireland, said in a statement it understood McKee was killed accidentally by a “Republican Volunteer.”

Leona O’Neill, a local journalist at the scene of the shooting, said that after McKee was hit and fell beside a police Land Rover, officers rushed her to Altnagelvin Hospital, where she died.

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Videos posted on social media showed police vehicles being pelted with what O’Neill said were dozens of petrol bombs, bricks, bottles and fireworks.

“NO ONE WANTS TO GO BACK”

McKee’s death was described as a tragedy by U.S. House of Representatives Speaker Nancy Pelosi, who led a congressional delegation to Londonderry to mark the 21st anniversary of the 1998 peace deal which Washington helped broker.

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The deal led to the decommissioning of weapons held by the main militant groups, the withdrawal of the British army from Northern Ireland and the creation of a power-sharing government.

But the detonation of a large car bomb outside a courthouse in Londonderry in January highlighted the threat still posed by pro-Ireland militants who rejected the Good Friday Agreement.

The groups, which are far smaller than those that operated before the peace deal and have very limited public support, have previously carried out attacks during the Easter period.

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

“Those who brought guns onto our streets in the ‘70s, ‘80s & ‘90s were wrong,” Democratic Unionist Party (DUP) leader Arlene Foster said on Twitter. “It is equally wrong in 2019. No one wants to go back.”