Northern Irish police arrested four men on Tuesday as part of an investigation into the murder of journalist Lyra McKee, whose killing sparked outrage in the province where a 1998 peace deal mostly ended three decades of sectarian violence.

The men, aged 20, 27, 29 and 52, were arrested in Londonderry under the Terrorism Act over the shooting in April 2019 that was claimed by Irish nationalist fighters opposed to the Good Friday peace deal.

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The New IRA, one of a small number of groups that oppose the peace accord, said one of its members shot the 29-year-old reporter dead in Londonderry when they opened fire in the direction of police during a riot McKee was watching.

Her death was condemned at the time by both Irish Catholic nationalist and pro-British Protestant unionist politicians.

Then-Prime Minister Theresa May described the attack as "shocking and truly senseless".

"I have always believed that some people within the community know what happened and who was involved," said Detective Superintendent Jason Murphy in a statement after Tuesday's arrests.

Murphy said he understood that people might be afraid to speak to the police but he promised people coming forward "anonymity for the purpose of this investigation", appealing for any mobile phone footage of the incident.

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 violence in Northern Ireland that largely ended with the 1998 deal.

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