Families of six men murdered by loyalist gunmen in a pub in Co Down have welcomed a watchdog investigation that exposed significant police collusion with the killers for finally delivering "the truth".

Northern Ireland's Police Ombudsman Michael Maguire was damning in his assessment of the police role in the 1994 Ulster Volunteer Force massacre in Loughinisland, Co Down.

Two UVF gunmen burst into the packed bar at around 10.10pm on 18 June 1994 and fired at customers watching the Republic of Ireland play Italy in the World Cup in the USA.

Six were killed and five injured.

Those murdered were Adrian Rogan, 34; Malcolm Jenkinson, 53; Daniel McCreanor, 59; Patrick O'Hare, 35; Eamon Byrne, 39; and Barney Greene, who at the age of 87 was one of the oldest victim of the Troubles.

Dr Maguire found that one man suspected of carrying out the mass killing in the Heights Bar was a police informant.

The ombudsman also said the murder squad had been involved in a number of other killings in the years beforehand, but had avoided arrest because the Royal Ulster Constabulary's Special Branch intelligence unit had withheld evidence from RUC detectives investigating the crimes.

He said some Special Branch officers had a "hear no evil, speak no evil, see no evil" mindset that placed the collection of information above the detection of crime.

Emma Rogan, whose father Adrian was killed, said: "Today we finally have a report by the Police Ombudsman that at last vindicates our long-held suspicions and belief that the truth about these murders was being covered up by the very people - the police - who were supposed to be protecting us, be on our side and investigate and bring to justice those responsible."

Barney Green's niece Noira Casement told a news conference hosted by the victims and victims' families following the report: "We have got the truth." 

Paddy McCreanor, nephew of victim Daniel McCreanor, said: "Collusion is no illusion and collusion happened. The truth has come out and that's all we ever wanted."

The families' lawyer Niall Murphy said the scale of the collusion was "terrifying".

"This report is one of the most damning expositions of state collusion in mass murder that has ever been published," he said.

While the report was revealed in Belfast, former prime minister John Major was elsewhere in Northern Ireland delivering a pro-European Union speech. Mr Murphy called on Mr Major to apologise for the collusion that took place when he was in office.

The families also called on current Northern Ireland Secretary of State Theresa Villiers to say sorry for referencing Loughinisland in a speech earlier this year about what she claimed was a "pernicious counter-narrative" of the Troubles that was trying to place undue blame on the security forces.

"We call on her to retract and apologise to us today," said Ms Rogan.

In his report's conclusion, Dr Maguire said he had "no hesitation in saying collusion was a significant feature of the Loughinisland murders".

No-one has ever been brought to justice for Loughinisland.

While acknowledging some officers worked tirelessly to catch the killers, the ombudsman described failures in the overall investigation as "catastrophic" and said in too many instances the probe was characterised by "incompetence, indifference and neglect".

The ombudsman also examined the role of police informants in efforts by loyalist paramilitaries to smuggle weapons into Northern Ireland from abroad during the mid to late 1980s.

He found that security forces were monitoring the importation bids and information provided by state agents in high positions within the paramilitary groupings helped lead to the recovery of a significant number of weapons.

But the watchdog questioned why many still ended up in the hands of loyalist killers.

He said unrecovered weapons from the shipments were used in 70 murders and attempted murders, including those at Loughinisland.

The ombudsman said there was no evidence the security forces had forewarning of the Loughinisland attack.

He said that after the murders, Special Branch did not appear to use sources in an effort to catch the killers.

"I have found no evidence that sources were tasked with gathering specific information that could have assisted the murder investigation," he said.

"This was a 'hear no evil, speak no evil, see no evil' approach to the use of some informants, which potentially frustrated the police investigation."

Dr Maguire said many officers in the RUC and their successors in the Police Service of Northern Ireland worked tirelessly to catch the killers, but he also identified a series of major failures in the investigation.

A report by previous police ombudsman Al Hutchinson in 2011 found that the RUC failed to properly investigate what happened in Loughinisland but said there was insufficient evidence of collusion.

Those findings were quashed after a legal challenge by relatives of those killed and Dr Maguire undertook a fresh investigation.

PSNI Chief Constable George Hamilton accepted the report's findings and said officers involved in collusion should be held to account.

"This report makes uncomfortable reading, particularly in relation to the alleged actions of police officers at the time," he said.

"The ombudsman has stated that collusion was a feature of these murders in that there were both wilful and passive acts carried out by police officers. This is totally unacceptable and those responsible should be held accountable."

He added: "These were appalling murders carried out by those with evil intent and I am very aware of the hurt and anger felt by the families of those killed and those injured. The PSNI remains firmly committed to apprehending those responsible for these murders and appeal to the community for information to allow us to do so."