An Historical Enquiries Team document clears the father of IRA victims' campaigner Willie Frazer of any terrorist involvement.
In the document obtained by the Belfast Telegraph, the HET "categorically states" that it has found "no evidence or intelligence" that the part-time UDR man was involved in loyalist paramilitary activity.
Bertie Frazer was accused in a Ministry of Defence (MoD) document published last weekend of involvement in a double murder in South Armagh.
Sunday World revealed details of the MoD report claiming the UDR man was part of a bogus checkpoint at which two Catholics returning from a GAA match were slaughtered.
Colm McCartney (22) and Sean Farmer (32) were shot dead near Tullyvallen on August 24, 1975.
Bertie Frazer was killed by the IRA six days later. Today is the 42nd anniversary of his death.
The MoD report, which will feature in a legal action brought by the families of loyalist victims, names him as one of the Catholic men's killers.
It also links him and at least 25 other security force members to the UVF's infamous Glenanne Gang, which it says was involved in more than 130 murders.
However, correspondence from the HET - which had been investigating Bertie Frazer's murder - stands in conflict with the MoD paper's claims.
In a letter dated August 29, 2012, a HET officer wrote to Willie Frazer: "I can state categorically that the HET review process has found no evidence or intelligence to suggest that James Albert Frazer was a member of, or associated with, any paramilitary organisation, loyalist or otherwise.
"The HET has found no evidence or intelligence to suggest that James Albert Frazer was involved in any terrorist activity."
The Frazers had themselves raised the issue of rumours of their father's alleged paramilitary involvement with the HET.
The senior investigating officer wrote: "One significant issue for your family is that of your father's perceived character and you have asked that this be specifically addressed as it is not reflected in the HET report.
"I am pleased to address this now but I would like to point out that the reason that this is not dealt with in the HET report is, quite simply, that your father's character has never been subject to suspicion."
Willie Frazer last night said his family had raised the issue with the HET because a nationalist politician had claimed that local UDR men were involved in the 1975 double murder.
He said: "My father was a well-known UDR man and we lived just two miles from where Colm McCartney and Sean Farmer were killed.
"So we felt it was being implied that he was involved in the murders, especially since the IRA shot my father dead less than a week later.
"We raised it with the HET because we wanted the matter cleared up.
"They checked it out and came back and said there was never any question of believing he was involved with terrorism. We as a family have nothing to hide."
Willie Frazer stated that his father had been at home on the night of the double murder.
"He is not here to defend himself, but we are not going to stand by and let his good name be tramped into the muck," he said. "My father hadn't a bad bone in his body. I have eight brothers and sisters and they are all upset about this, especially my youngest sister."
Willie Frazer said that his solicitor would be asking the MoD for a copy of the document and he called on the Sunday World to publish it.
"Let them put it out there in the open and whatever's in it, I'll take on the chin. If his name is being blackened, it is only right that we can at least look at the material," he said.
"I've had death threats from republicans long before this, but they've increased in recent days with those making the threats saying, 'Your Da's a killer'.
"These allegations have no foundation. The Mid-Ulster UVF was behind the shooting, my father played no role in it."
Mr Frazer said that, after his father's death, his mother had received hundreds of Mass cards from Catholics. He offered to meet relatives of Colm McCartney and Sean Farmer to reassure them "that Bertie Frazer played no part in their murder".