The Police Ombudsman has dismissed claims detectives protected a suspected IRA killer from being charged with murder because he was an informer.
Investigators also found nothing to suggest police had prior evidence which could have prevented the 1984 murder of Mary Travers, 22, in south Belfast.
Resident Magistrate Tom Travers and his family were attacked as they left Mass. He was shot six times but survived.
Nuala O'Loan criticised Special Branch for not passing on all information.
A woman was convicted of the attack, but a man known as Man A was acquitted of the same charges.
Mr Travers had identified him from police photographs and an informal ID parade as the man who shot him, and 20 years later a Sunday newspaper suggested he was a Special Branch informant and a "protected species".
It was suggested the RUC had colluded with him by allowing him to commit murder while being protected from the full rigours of the law.
Mr Travers asked the Police Ombudsman to investigate the claim but later withdrew the request.
However, Mrs O'Loan decided it would be in the public interest to complete the investigation.
"While I cannot confirm or deny whether Man A was an informant, I have found no evidence that he was protected from the law," she said.
She said she was satisfied police had made substantial efforts to bring the perpetrators of the attack to justice.
Although she did not find that Special Branch had frustrated efforts to gather evidence from a search of a suspected safe house, she said it "failed to share all relevant intelligence with the murder investigation team".
She added: "It is also totally unacceptable that two guns used in this attack have since either been lost or destroyed."
She revealed a file has been sent to the Public Prosecution Service regarding a former RUC officer alleged to have told Mr Travers he failed to secure evidence from a location following the murder.
Mrs O'Loan said the service was considering whether the former officer should face a charge of Misconduct in Public Office.
The PSNI said it was considering the Police Ombudsman's recommendations, adding that the Historical Enquiries Team is "progressing lines of inquiry" as part of their review.
SDLP assembly member Alban Maginness said questions remained unanswered about the RUC's murder inquiry.
"It seems to me reading this report that there was a litany of mistakes made by the police in the investigation, and a considerable amount of incompetence," he said.