A report by Northern Ireland's Police Ombudsman into the murder of an RUC sergeant in 1977 has concluded that senior officers in the force failed to warn their colleague that his life was under threat.

SDLP leader Dr Alasdair McDonnell has said he knew the victim, Sgt Joe Campbell, and described Ombudsman Dr Michael Maguire's report as "explosive".

RUC Sgt Campbell, a father-of-eight, was killed by a single rifle shot as he locked up the RUC station in the quiet Co Antrim seaside town of Cushendall in 1977.

The son of a garda from Co Monaghan, he was a Catholic and was widely-respected everywhere he served.

Republicans were not suspected of his murder.

For years the victim's family have alleged there was security forces collusion and that a Loyalist gunman was involved.

Twelve years ago, the Campbells requested the police ombudsman to investigate. His 50-page report is published today.

It does not speculate who specifically carried out the murder.

However, it says that senior officers, right up to the very top, failed to warn Sgt Campbell that his life was under threat.

Many former RUC officers gave evidence that influenced the ombudsman's assessment.

The controversial findings are published on the day that Matt Baggott steps down as PSNI chief constable after five years in the job.

Mr Campbell's son said his family are unhappy that the report into his father's murder does not bring any clarity to events surrounding his father's death.

Speaking on RTÉ's News at One, Joe Campbell said after lengthy investigation public officials cannot shed light on his father's death.

"It's quite shameful and distressing for the family that they did nothing about it. In fact we will say they colluded in his murder," he said.

"An additional sad fact is that some of those people are still alive today and they refused to cooperate with the ombudsman's inquiry.

"There are many who did and we thank them, but those who didn't cooperate they have to look at themselves and ask what kind of a future do they want for their children and grandchildren," Mr Campbell added.