A report by the Police Ombudsman that said there was collusion between some RUC officers and UVF gunmen who killed six Catholics in Loughinisland 23 years ago was challenged in court on Friday.
Two retired officers are attempting to have the report quashed.
A lawyer acting for them claimed the Ombudsman, Dr Michael Maguire, did not have the legal authority to reach such a conclusion.
Families of those killed have described the legal action as an insult.
The families always said the gunmen had not acted alone.
They claimed the killers were helped by police officers.
Last year, the Police Ombudsman agreed.
"I have no hesitation in saying that collusion was a significant feature in the Loughinisland attack," he said at a press conference to launch his report into the atrocity.
Two retired police officers, one of whom was referred to but not named in the report, are mounting a judicial review of the report.
They claim the Ombudsman acted beyond his legal powers.
Their lawyer, David McMillen QC, told the High Court in Belfast that the Ombudsman had failed to follow proper procedures.
He said Dr Maguire did not have the legal authority to investigate the killings in the way he had, or to publish a report stating that former police officers had committed criminal offences.
"The headline finding was that there was collusion," he said.
"It is hard to imagine a more inflammatory or damning indictment by a public body against a police force."
Mr McMillen said there had been a finding of serious criminal activity by individuals "damned in the minds of the public without the right to a fair hearing".
He said the Police Ombudsman appeared not to understand how he was supposed to operate because he had failed to follow statutory guidelines.
A lawyer acting for the Police Ombudsman disputed those claims.
He said the legislation governing the office meant he had the latitude to act in the way he had.
"If the applicants are right in their submission, then what they are saying is that the Ombudsman should have simply walked away and done nothing about the serious allegations of collusion," said Tony McGleenan QC.
"That isn't right. The seriousness of the allegations meant the ombudsman had no option other than to investigate."
Relatives of some of those killed in the Heights Bar in June 1994 were in court to hear the legal arguments.
Afterwards one of them, Emma Rogan, whose father Adrian was one of victims, criticised those mounting the legal challenge.
"The only person that ever gave us any sort of truth was the Police Ombudsman, and here today we are back in court again with the police officers, the very people that were supposed to investigate these murders at the time in 1994, are trying to have that report quashed," she said.
"It's just re-victimising families and traumatising people."
Solicitor Niall Murphy, who represents the families of the victims, described the legal action as an insult to them.
"This is an application taken by retired police officers, many of whom did not co-operate with the Police Ombudsman's investigation," he said.
"This report has been accepted in its entirely by the Chief Constable, by the then Secretary of State Theresa Villiers, and also the prime minister, who wrote to the families on the 12th of July last year to say that the British government accepted the report wholly, in its entirety."
The hearing will resume again next week.