The unsolved murder of a Catholic police officer shot dead in Northern Ireland more than 40 years ago has been linked to a loyalist paramilitary group responsible for some of the worst atrocities of the Troubles.

At the preliminary hearing of a legacy inquest into the 1977 killing of Sgt Joseph Campbell in Belfast on Thursday, a solicitor representing the police ombudsman told Justice Siobhan Keegan she would be disclosing files of sensitive material to the coroner’s office to assist its inquiry into the murder of the Royal Ulster Constabulary officer and father of eight Campbell.

Louisa Fee said the material had been gathered during Operation Newham, the ombudsman’s investigation into the activities of the mid-Ulster UVF and the Glenanne Gang, a loyalist terrorist group that included several members of British security forces and has been linked to 120 murders including the Miami Showband massacre.

Rosemary Campbell, the police officer’s 84-year-old widow, has been campaigning for 40 years for government agencies to reveal details of her husband’s death. She told the Guardian she was hopeful the inquiry would finally deliver justice to the family.

“It has been a horrible 42 years. It will be groundbreaking just to get the details of the truth. Everyone has a right to know how their loved one died,” she said.

Campbell was shot outside Cushendall police station on 25 February 1977. He had been warned in threatening phone calls that his life was at risk, and fellow police officers told senior figures in the RUC that loyalist paramilitaries had targeted him to be killed. Two murder investigations have failed to find his killer.

The attorney general ordered a fresh inquiry into the murder in 2016 after being presented with evidence in a police ombudsman’s report that concluded senior RUC commanders could have prevented Campbell’s murder. The ombudsman also reported that his investigation was impeded by a lack of police cooperation.

“My investigation was hampered by both the refusal of a number of retired senior police officers to cooperate and the loss of police documentation,” the ombudsman, Michael Maguire, wrote in 2014.

“Police failed to act on information available to them and by doing so placed Sgt Campbell’s life in danger.”

Maguire’s report states that “much of the investigative activity associated with this tragic event has been corrupted” before concluding he could not uphold the Campbell family’s complaint that police suppressed evidence indicating that the gunman was prolific loyalist killer and British military intelligence and Special Branch agent Robin Jackson, assisted by RUC officers. Jackson has been named as a key member of the Glenanne Gang.

“The public interest in this case cannot be overstated,” said Fearghál Shiels, the solicitor representing the Campbell family.

Jackson, known as The Jackal, is thought to have been responsible for more than 100 murders of mostly Catholic civilians. He was never brought in for questioning in relation to Campbell’s murder and died of cancer at his home in 1998.

The Campbell inquest is one of more than 50 legacy inquest cases into almost 100 killings currently under way in Northern Ireland, which the lord chief justice Sir Declan Morgan has committed £55m to deal with over six years. The presiding coroner, Justice Siobhan Keegan, is currently holding preliminary hearings on all outstanding legacy cases to consider their readiness for hearing.

Rosemary Campbell is separately suing the Police Service of Northern Ireland for the failure to prevent her husband’s death.

“So much evidence has been hidden, papers have gone missing but at a public inquest surely some of this evidence will have to come out,” she said.

Assistant chief constable George Clarke told the Guardian that the PSNI would cooperate fully with the coroner’s inquest.

He said: “We [have] acknowledged that there were significant shortcomings in the RUC handling of information prior to the murder and in both subsequent police investigations into Sergeant Campbell’s murder. We have previously apologised for that and reiterate that apology.”

The preliminary hearings of legacy cases will continue until 4 October.