Police did not have any advance warning that could have stopped the Greysteel massacre, the Police Ombudsman has said.
Nuala O'Loan rejected a complaint by MLA John Dallat about police actions before the UDA "trick or treat" attack that left eight people dead in 1993.
But in keeping with the legal constraints upon her office, she could not confirm or deny Mr Dallat's allegations that killer Torrens Knight was a security force agent at the time.
Mr Dallat alleged last year that he had given the RUC information in 1993 that could have prevented the Greysteel attack.
He said he had passed on the information after the UDA killed four Catholic workmen - one an IRA member - in March 1993.
Mrs O'Loan said her investigators found police had taken "appropriate action" on Mr Dallat's information and that there was no evidence that that information could have prevented the attack on the Rising Sun bar.
"Unfortunately Mr Dallat was unable to provide supporting information, either in terms of copies of documents or details of the times, places or the exact person to whom he provided each piece of information," the Ombudsman said.
"I understand Mr Dallat may have felt that to keep such records could have compromised his safety, but the lack of such detailed evidence impacted upon the investigation."
Mrs O'Loan said three retired officers were mentioned by Mr Dallat. She said they cooperated with her probe, but had no recollection of Mr Dallat providing information that could have prevented the Greysteel attack.
The Ombudsman said the question of whether Torrens Knight was an agent was not within her remit, but she found no evidence that he was protected by police.
Mr Dallat said: "Relatives of the deceased did want to know if anything could have been done to prevent their killings. From my point of view I regret that the police did not have greater success and given the lack of documents available to the Police Ombudsman there is no indication of what they did to avert it. I regret that."
He added: "Obviously I would want to put on record that my contact with all three officers was a lot more frequent and detailed than their memories serve them.
"Also missing is any detail of what steps were taken to catch the killers earlier, although I do accept that their 'den' was searched on many occasions, but to suggest that it was because it was a 'drinking den' is far off the mark."