A former police officer of the Royal Ulster Constabulary (RUC) has been referred to public prosecutors in Northern Ireland in relation to a series of potential offences including the sectarian murder of three brothers.

John Martin, 24, Brian, 22, and their 17-year-old brother, Anthony Reavey, were shot by the notorious loyalist Glenanne gang at their home in County Armagh in 1976.

The referral by the Police Ombudsman comes two months after the UK government proposed an amnesty for all Troubles-related offences, something that is opposed by the five main political parties in Northern Ireland, victims’ groups and human rights organisations. On Thursday, the Council of Europe told the Northern Ireland secretary, Brandon Lewis, that the plan to end Troubles-related prosecutions could breach international law.

The Glenanne gang was a secret alliance of loyalists and included former British soldiers in the Ulster Defence Regiment, RUC officers and members of the paramilitary Ulster Volunteer Force.

It is linked to 120 murders in Northern Ireland and has been the subject of several investigations leading to the 2006 Cassel report and three reports commissioned by the Irish supreme court judge Henry Barron.

Its members were suspected of involvement in the 1974 Dublin and Monaghan bombings that killed 33 people, and the 1975 Miami Showband massacre – one of Ireland’s best known bands in the early 1970s.

Allegations of collusion between the gang and state forces are also the focus of one of several high-profile investigations into killings during the Troubles by the former Bedfordshire police chief Jon Boutcher.

A file on the former RUC officer was sent to the Public Prosecution Service on Monday after new evidence came to light. “The Police Ombudsman’s office has submitted a prosecutorial advice file to the Public Prosecution Service in relation to potential offences by a former RUC officer in the 1970s,” the ombudsman’s office said in a statement. “The potential offences include the murders of brothers John, Anthony and Brian Reavey at Whitecross, County Armagh, in 1976.”

The victims’ brother Eugene Reavey said the development was “a major breakthrough for me and my family” more than 40 years after their deaths. “When they told me they had received fresh evidence, and that they would be referring the matter for prosecution, I couldn’t believe it,” he told the Irish News. “I promised my father that I would continue the fight for justice for John Martin, Brian and Anthony, and today I believe we are a step closer to that.

“It is regrettable that the police did not do their job sooner,” he said. “This is 46 years in the making. I told them 40 years ago who murdered my brothers and they never gave me any heed. I now look forward to seeing those responsible in court and held to account for their actions.”