Detectives did not have information which could have prevented the 1993 murders of eight people in Greysteel, the Police Ombudsman has found.
Eight people were killed in the Rising Sun massacre
SDLP MLA John Dallat had made a complaint, saying crucial information he gave to detectives was not acted on.
Nuala O'Loan also found there was no evidence killer Torrens Knight was protected from the rigours of the law.
Eight people were shot dead when a loyalist UFF gang opened fire inside a bar in the County Londonderry village.
It happened at the Rising Sun bar at Halloween 14 years ago.
Mr Dallat said the report put on record his efforts to bring the Ulster Freedom Fighters gang to justice before the atrocity happened.
One of the gunmen shouted "trick or treat" before opening fire on customers in the Rising Sun bar.
One of the gunmen shouted "trick or treat" before opening fire
Four UFF men, including Knight, were given eight life sentences for the murders.
Mrs O'Loan said the issue of whether Knight was a police informant as alleged was not within her remit, but the fact he was convicted of the Greysteel and Castlerock murders suggested he was not being protected.
In March 1993, four workmen were shot dead at Castlerock by the UFF.
Mr Dallat complained to the Police Ombudsman that information he gave to police after this shooting could have prevented the Greysteel murders seven months later.
Police Ombudsman investigators established that Mr Dallat had given officers some information and they had taken appropriate action.
However, Mrs O'Loan found these details could not have prevented the Greysteel attack.
The three officers mentioned in Mr Dallat's complaint are now retired.
Mrs O'Loan said they all fully cooperated with the investigation and remembered meeting the SDLP assembly member.
"However, when asked if Mr Dallat provided information to them during the period 1991-1993 which could have help prevent the terrorist attacks at Castlerock and Greysteel, they each say they have no recollection of this," she said.
In response, Mr Dallat insisted that his contact with the officers "was a lot more frequent and detailed than their memories serve them".
"Although the police may not have had the evidence to arrest the killers after Castlerock, they certainly had their suspicions and this was understood in any conversations I had with any of the three officers," he said.