The family of a nationalist councillor murdered in Co Tyrone in 1974 were failed by police as a result of "latent investigative bias" and a "wholly inadequate investigation", Northern Ireland's Police Ombudsman has found.
The report accuses RUC Special Branch of "deliberately turning a blind eye" to intelligence linking British soldiers and police officers to a UVF gang.
Patsy Kelly, a 35-year-old father of five, was abducted and shot dead in July 1974 shortly after locking up the bar where he worked in the village of Trillick.
His body was recovered from a remote lake 24 kilometres away in Co Fermanagh just over two weeks later.
It had floated to the surface despite being weighed down by his killers, who shot him six times.
His family alleged that he was killed by members of the Ulster Defence Regiment (UDR) and that a number of named suspects were not arrested because they were soldiers.
In a 134-page report, Police Ombudsman Marie Anderson said there was evidence of "collusive behaviour".
She said RUC Special Branch had withheld information from the police investigation team, including intelligence that a UVF unit in Co Fermanagh was being assisted in its activities by identified members of the UDR and RUC.
The report said that at an early stage, the senior investigating officer considered the involvement of UDR members in the murder as "a key line of enquiry".
However, the Ombudsman said that while 20 UDR members were interviewed or gave witness statements, the majority of their alibis for their whereabouts on the night of the murder were not checked.
Ms Anderson says the potential involvement of UDR members was discounted at an early stage as a result of "'latent' investigative bias."
"My investigation has established a failure to address the implications of other intelligence linking RUC and UDR members to a UVF unit based in County Fermanagh," Ms Anderson said.
"I am of the view that this was indicative of deliberately 'turning a blind eye' which constituted 'collusive behaviour' on the part of the RUC Special Branch," she added.
A memorial plaque and crucifix mark the spot where his family believe he was killed by members of the UDR a short distance from the bar on a road leading to his family home.
Local people at the time reported seeing a UDR checkpoint on the road and the sound of shots being fired.
Blood spatters, cartridge cases and shirt buttons were discovered at the scene later that day.
No one has ever been charged or convicted in connection with the killing.
Loyalist paramilitaries claimed they were responsible for the murder, but neither the police or local nationalists believed them.
The family believe evidence was withheld from the original police investigation and that a number of named suspects were not questioned because they were members of the UDR.
A few days after Mr Kelly disappeared an anonymous letter was sent to the local civil rights movement naming local UDR soldiers as being involved, but they were not arrested or fingerprinted.
As the letter was written on the back of an RUC accident report form, there was speculation that the author was a serving police officer with knowledge of the investigation.
In the late 1990s, a former member of the UDR in the area told people in a local pub that he had witnessed the killing and named a number of former soldiers he served with who he claimed were responsible.
One of those he named was also named in the anonymous letter sent to the civil rights movement.
The former soldier was never questioned by police about his claims and died in 1999.
Details of forensic evidence were also withheld from Mr Kelly's inquest and his family.
It included a footprint found at the scene where his family believe he was shot dead. Forensic testing concluded that the print matched the print made by boots issued to members of the British Army at the time.
Neither were they informed about fingerprints found on Mr Kelly's car, which was found around 16km away near Brookeborough in Co Fermanagh a few days after he was last seen alive.
Patsy Kelly Jnr, who now lives in Co Kerry, was born eight months after his father was killed.
His mother Teresa only discovered she was pregnant just days after Mr Kelly went missing.
He drove from Kerry to join his mother and two brothers at the meeting.
"It has been a long road to reach this particular juncture in our campaign and we hope that this report will in some way help to reveal the full truth of what happened to a much-loved husband and father," he said.
"Our campaign has one simple goal and that is that after almost 50 years that the full truth of the events surrounding Patsy's abduction, murder and subsequent cover-up will finally be revealed."
The report is being published as controversial British government legislation to end prosecutions for Troubles killings is going through the House of Lords.
Critics say the proposed new laws are designed primarily to prevent potential prosecutions of British military veterans, including former members of the UDR.
