Police investigating the killing of journalist Lyra McKee, who was shot dead by paramilitary gunfire in Northern Ireland last month, have made four arrests.

The Police Service of Northern Ireland says males 15, 18, 38 and 51 years old were arrested in Londonderry, also known as Derry, on Thursday under terrorism legislation, in connection with violence in the city the night McKee died.

McKee, 29, was watching rioting on April 18 when she was killed. Hours before her death, she tweeted a photo of the riot with the words: “Derry tonight. Absolute madness.”

Police told Reuters they did not believe McKee was working at the time of the attack.

Dissident paramilitary group the New IRA, which opposes Northern Ireland’s peace process, says its members killed her by accident when firing at police.

A local journalist at the scene of the shooting, Leona O’Neill, wrote on Twitter that after McKee was hit and fell beside a police Land Rover, officers rushed her to hospital, where she died. Videos posted on social media showed police vehicles being pelted with what O’Neill said were dozens of petrol bombs, bricks, bottles and fireworks.

McKee rose to prominence in 2014 with a blog post describing the struggle of growing up gay in Belfast. She was writing a book on the disappearance of young people during the three decades of sectarian violence in Northern Ireland known as The Troubles, which ended with the Good Friday agreement in 1998.

McKee’s death caused wide shock and grief in the region. Three people had earlier been arrested and released without charge.