LONDON >> Police in Northern Ireland searched for multiple suspects today after the fatal shooting of a journalist during rioting in Londonderry and sought help from the public to get “a killer off the streets” and into custody.

The Police Service of Northern Ireland said 29-year-old journalist Lyra McKee was shot and killed, probably by a stray bullet, during overnight rioting in the city’s Creggan neighborhood. It said the New IRA dissident group was most likely responsible.

Assistant Chief Constable Mark Hamilton said a gunman fired a number of shots at police during the unrest that began Thursday evening.

“We believe this to be a terrorist act,” he said.

Police today released closed-circuit TV footage showing the man suspected of firing the shots that killed McKee.

Detective Superintendent Jason Murphy said the footage shows “the gunman at the corner and an individual picking up something from the ground on the same corner. We are releasing this to encourage anyone with information to make contact with us.”

He said locals know the identity of the gunman and urged them to come forward “to try to help us take a killer off the streets.”

The killing reminded many of the decades of violence that plagued Northern Ireland before the 1998 Good Friday peace agreement. It was condemned by all the major political parties as well as the prime ministers of Britain and Ireland.

Speaking in Dublin, Irish Prime Minister Leo Varadkar said the people of Ireland and Northern Ireland had chosen peace and cooperation on Good Friday 21 years ago and will not be “dragged into the past” by political violence.

McKee rose to prominence in 2014 with a moving blog post — “Letter to my 14 year old self” — describing the struggle of growing up gay in Belfast, the capital of Northern Ireland.

In the post, she described the shame she felt at 14 as she kept the “secret” of being gay from her family and friends and the love she eventually received when she was finally able to reveal it.

She also had recently signed a contract to write two books.

Hours before her death, she tweeted a photo of the rioting with the words: “Derry tonight. Absolute madness.”

Her partner, Sara Canning, told a vigil today that McKee’s amazing potential had been snuffed out.

Canning said the senseless murder “has left me without the love of my life, the woman I was planning to grow old with.”

“It has left so many friends without their confidante,” she added.

A murder investigation has been launched but there have been no arrests yet. Police appealed for calm over the long Easter holiday weekend.

Hamilton said the force’s assessment “is that the New IRA are most likely to be the ones behind this.”

The New IRA is a small group who reject the 1998 Good Friday agreement that marked the Irish Republican Army’s embrace of a political solution to the long-running violence known as “The Troubles” that claimed more than 3,700 lives.

The group is also blamed for a Londonderry car bombing that did not cause any injuries in January. It is regarded as the largest of the splinter dissident groups still operating and has been linked to several other killings in the past decade.

Deputy Chief Constable Stephen Martin said police believe more than one person was involved in the shooting.

“We certainly believe there was more than one person who was involved in this last night. Obviously only one person pulled the trigger but there was more than one person,” he said.

He said the violence started after police entered the area to search for weapons and that the gunman was aiming at policemen when the rioting intensified.

“The full and total responsibility for Lyra McKee’s death lies with the organization that sent someone out with a gun,” he said.

There has been an increase in tensions in Northern Ireland in recent months with sporadic violence, much of it focused in Londonderry, also known as Derry.

Londonderry Mayor John Boyle said the city was united in mourning McKee’s death.

“I have known her since she was 16 years old,” he said. “She was bright, she was warm, she was witty. But most of all, she was an outstanding individual, a great friend to so, so many people in this city in the short time that she was with us.”