Northern Ireland police said Friday the attempted murder of a senior off-duty officer was "terrorist-related", accusing the New IRA republican splinter group of the shooting.

Two men opened fire on Detective Chief Inspector John Caldwell on Wednesday while he coached children, including his son, in football in the town of Omagh in County Tyrone.

"John remains in hospital, where he is critically ill and heavily sedated," PSNI Chief Constable Simon Byrne told journalists.

"I can confirm that we are now treating it as terrorist-related and our primary line of inquiry is the New IRA," added Assistant Chief Constable Mark McEwan with the investigation focusing on the splinter group of the Irish Republican Army.

A fourth man aged 22 years was arrested on Friday after three aged 38, 45 and 47 were taken into police custody on Thursday.

Police in Northern Ireland are the target of sporadic attacks that were once common in the region, which was plagued by decades of sectarian violence before the signing of 1998 peace accords.

The New IRA has admitted responsibility for two attacks in recent years. In April 2021, a bomb was planted under a policewoman's car in front of her home.

On Friday, Sinn Fein's deputy leader Michelle O'Neill described the attempted murder of Caldwell, a high-profile officer who has led major investigations, as "an attack on all of us, this is an attack on our community.

"I think that the most powerful message that we, as political leaders (can do, is) to stand with the chief constable today, to stand with the police service and say, this is not good enough," she added.

Tensions are running high in the province after unionists loyal to the UK collapsed its power-sharing government to protest post-Brexit trading rules. They say these rules are distancing Northern Ireland from the rest of the UK.

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