No-one was protected from prosecution during a police investigation into one of the worst atrocities of the Troubles, a report has found.
There was also no prior intelligence that could have prevented the La Mon bombing in February 1978, which killed 12 people and injured 23.
There had been suggestions that police agents among the IRA bombers responsible might have been protected during the investigation.
But a report by Northern Ireland's Police Ombudsman said there was nothing to support that contention.
The ombudsman had said concerns were raised that informants may have been involved in the bombing, and so the RUC may have had information which may have prevented the atrocity or that informants may have been protected by police.
Relatives of those murdered in the bombing have been briefed on the findings of the investigation.
Twelve people were killed and 23 injured when a blast incendiary hung on the windows of a hotel function room at the venue exploded on 17 February 1978.
Guests attending the Irish Collie Club annual dinner dance were engulfed in a huge fireball.
Senior Director of Investigations with the Police Ombudsman's Office, Paul Holmes, said the investigation did not see evidence or intelligence that would support the suggestion of malpractice by police officers and found no intelligence which could have forewarned of or prevented the bombing.
He said: "The substantial police investigation files, associated documentation and intelligence records reviewed by this office, together with explanatory accounts from a number of former police officers and other enquiries, lead me to conclude that there was earnest endeavour by the RUC to bring those responsible for the bombing of La Mon House to justice.
"However, maintenance of the investigation did not have the same rigour and this manifested in the loss of investigative material and the failure to interview the person arrested in 1991 in relation to the bombing. I am of the view, based on all available evidence and information, that collusive behaviours were not a feature of this RUC investigation."
"The evidence and intelligence viewed by the Police Ombudsman's investigation attributes responsibility for directing, enabling and perpetrating the bombing of La Mon House to the Provisional IRA," Mr Holmes said.
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The ombudsman also found that the original investigation was compromised after a man's confessions were deemed inadmissible by a trial judge.
The man alleged he had been assaulted by police while in custody.
The ombudsman has published a report into the atrocity, which considered the effectiveness of the investigation by the Royal Ulster Constabulary (RUC), including resulting prosecutions against two men, one who was convicted of 12 counts of manslaughter in 1981, and the other who was acquitted during his trial in 1980.
At the trial, the judge said he could not be satisfied beyond reasonable doubt that the man's statements had not been "adduced by torture or inhuman or degrading treatment" and the statements were excluded.
The case was dismissed in the absence of other evidence.
Mr Holmes said the judge's comments and trial outcomes point to "a violation of the rule of law by the police officers involved".
Lack of documents 'frustrating' - ombudsman
The ombudsman said that a loss of records and exhibits from the original investigation was "frustrating".
Mr Holmes said: "My ability to provide a definitive view on the effectiveness of the RUC investigation, including whether all appropriate lines of enquiry were pursued, has been hindered by the unavailability of certain documentation generated by the investigation.
"It has not been possible to establish the circumstances in which these records were lost. However, this is a recurring, systemic issue in legacy cases and is not unique to this investigation."
Ombudsman investigators were able to access the majority of records generated by the RUC investigation and concluded the initial response to the attack was prompt and well managed.
"The first police officers to attend the scene displayed courage and professionalism in seeking to save lives and tend to the injured," said Mr Holmes.
"More than 100 detectives were involved in the murder investigation and almost 400 statements were recorded.
"Scenes were identified and exhibits seized, and forensic examinations took place.
"In the context of the significant pressures and challenges faced by the RUC at that time, I am of the view that the investigation was well resourced and received direction from the highest levels of the RUC."
Ombudsman investigators also found that some enquiries appeared to not have been actioned by the RUC, including a failure to conduct enquiries regarding the second bomb warning call.
Additional reporting PA
