Police carried out a “comprehensive” investigation into the murder of Robert McCartney in Belfast, a watchdog said.
Nobody was protected in the probe into the killing of Mr McCartney in January 2005, Police Ombudsman Dr Michael Maguire added.
Mr McCartney, 33, was stabbed to death outside Magennis’s bar in Belfast, near his home. IRA members were suspected of involvement in the killing.
The ombudsman’s office said: “The police investigation of events that night was complex, with what can best be described as some unique obstacles, including a reluctance by some witnesses to give evidence and concerns about the credibility of others.
“The detectives sought to work around these problems. Their investigation was detailed and comprehensive and resulted in three people facing trial.”
Dr Maguire said that while the family of Mr McCartney may feel the criminal justice system has failed them, there is no evidence police played any part in that failure.
He added: “A lot of the public discussion on what happened has focused on events in Magennis’s bar, although Robert and his friends were subjected to a brutal attack in a busy street close to the city centre.
“The McCartney family may have comforted themselves with the thought that the very public nature of the attack would in itself have helped find and convict those responsible.
“I can only begin to understand their frustration when this did not happen.”
The Ombudsman’s Office received a series of complaints about how police conducted the investigation, including that those responsible may have been police informants who were protected from justice.
Dr Maguire said: “I can understand if the family feel the criminal justice system has failed them.
“Having examined all the information carefully, I can assure them that the fact that no one has been convicted for the murder can in no way be attributed to the work of police in gathering evidence.”
The ombudsman found that the police delay in arriving at the bar was not due to inefficiency. The victims had been discovered nearby and the priority for officers was the preservation of life, after which they could begin to seek to identify where the attacks happened.
His investigators established that the equivalent of three industrial bins of material was recovered from the scenes and extensive DNA testing was carried out on blood matter.
They found that, despite allegations to the contrary, police interviewed the man alleged to have cleaned the bar area after the murder.
It was also alleged that people were allowed to leave the scene without their names and addresses being recorded.
Ombudsman investigators confirmed that people left the bar before the arrival of police, who then ensured they got contact details for all those who were still present.
They said police had gone to considerable lengths to identify those who had been in the bar earlier.
The issue of efforts to find the origins and whereabouts of the knife used to stab Mr McCartney was also raised.
Investigators established that police interviewed several people and conducted an extensive search of drains and rooftops over a wide area in their efforts to find the weapon.