Police investigative bias led to the IRA being wrongly blamed for a Belfast pub bombing in which 15 people died, the Police Ombudsman in the North said today.
More than 16 others were injured in the Ulster Volunteer Force attack at the Catholic McGurk's bar in 1971.
It was initially wrongly viewed by security forces in the North as an IRA "own goal" - an accidental explosion - and police failed to conduct an effective investigation into whether loyalists were responsible, today's 80-page report from Al Hutchinson said.
The Royal Ulster Constabulary (RUC) was predisposed to point the finger at republicans and gave "selective and misleading" briefings to the British Government and media which turned out to be wrong, he added.
The Ombudsman concluded that the RUC investigation was not proportionate to the magnitude of the incident, which was one of the biggest losses of life during any incident in the conflict until the Omagh bombing in 1998.
Mr Hutchinson said: "The police investigation had a clear predisposition to the erroneous 'own goal' theory. This was perhaps in some way understandable given the extent of IRA bombings and attacks at the time.
"However, the investigative bias leading to a failure to examine properly evidence and intelligence attributing the bombing to loyalist paramilitaries undermined both the investigation and any confidence the bereaved families had in obtaining justice."
The 50lb no-warning device was left on the doorstep of the family-run bar in Great George's Street, north Belfast.
It exploded on the evening of Saturday December 4 1971, killing 15 people and injuring more than 16 others.
During the days which followed, the media carried speculation as to which terrorist group was responsible, including information attributed to police sources, blaming republican paramilitaries.
For four decades allegations of collusion and cover-up have surrounded the atrocity.
Just hours after the blast, the Army said the bomb was clearly inside the pub and it was an IRA "own goal", effectively detonated by accident.
Then-home affairs minister John Taylor said on television and in Stormont that the bomb was an IRA device. Mr Taylor, who said he was briefed by British Home Office staff, said there was no question that the bombing was a Protestant paramilitary operation.
A complaint made by representatives of some of the families of those killed to the Ombudsman focused on allegations that the RUC colluded in the attack, did not properly investigate it and provided false information to suggest that it was an "IRA own goal".
The Ombudsman's investigation found no evidence that members of the RUC assisted the passage of the terrorists in getting to or away from McGurk's Bar.
It acknowledged that the prevailing situation in the North at the time presented significant challenges to policing.
In particular Mr Hutchinson recognised that for police officers and other emergency services to come under sustained gun attack in the vicinity of the bombing, which left one man dead and others injured, frustrated the initial work of detectives.
The Ombudsman has established that initial intelligence and information which police received presented them with a confusing picture as to who had carried out the attack.
He found that, in the following weeks, despite emerging evidence supporting the alternative theory, the RUC became unduly influenced by information which suggested that republican paramilitaries had been responsible.
His report concluded that police failed to give adequate consideration to the possible involvement of loyalist paramilitaries.
It said the police's briefing to the British Government had been "selective and misleading".
Mr Hutchinson added: "It is entirely appropriate that the Government [British] is kept well-informed about critical incidents but it is crucial that such briefings are balanced and accurate.
"A more rigorous approach to the assessment of and reporting on such a catastrophic loss of life should have been adopted.
"Inconsistent police briefings, some of which inferred that victims of the bombing were culpable in the atrocity, caused the bereaved families great distress, which has continued for many years.
"Following the 1977 arrest and the 1978 conviction of a loyalist suspect, the RUC failed to conduct an effective investigation of the information, which it had received in 1976, that the convicted man and other members of the UVF had been responsible for the bombing."