A forensic review of the Clydach murders has made “significant findings” linking convicted killer David Morris to the crime scene, police have said.
Morris, who died in August aged 59, was convicted of killing four members of the same family in the Swansea Valley village in 1999.
South Wales Police said a link between him and a sock had been identified during an independent investigation.
The force added this was the first time DNA evidence had linked Morris to the murder scene in Kelvin Road.
The BBC has learned the family of David Morris are questioning the findings and do not accept them.
Morris was convicted in 2002 of killing Mandy Power, her daughters Katie and Emily and her mother Doris Dawson, who were bludgeoned to death at their home.
He was twice tried for murder and was serving a 32-year sentence when he died.
Last October, doubts were cast about the conviction as potential new witnesses and expert views emerged.
But South Wales Police said a scientific link between Morris and a sock, which it added was widely accepted as being used by the murderer during the killings, had been identified.
The force added that while a link to Morris – or a male relative of his paternal lineage – had been found, it cannot determine how or when Morris’s profile was transferred on to the sock.
Scientists found it was “more likely” Morris contributed to the DNA profile found on two different areas of the blood-stained sock, than if he did not contribute DNA to them.
Permission was granted to take a blood sample from Morris after his death on 20 August to allow forensic examinations to take place.
The technology used in the process would not have been available to the original investigating team, the force said.
The link was identified using Y-STR profiling, a technique which specifically targets male DNA, even in a sample which contains a mixture of male and female cellular material.
Assistant Chief Constable David Thorne, of South Wales Police, said: “The decision to carry out an investigative assessment did not constitute a reopening or reinvestigation of the murders, nor did it demonstrate any lack of confidence in the conviction of Morris and the subsequent case reviews.”
He added: “This is significant as the sock was recovered from the murder scene and it was widely accepted that it was used by the killer.
“The outcome of the forensic assessment and completion of further actions have not established any information that undermines the conviction of Morris.
“In my view, as the independent senior investigating officer, the new findings from the samples taken from the sock support the existing evidence that originally convicted him.”
Assistant Chief Constable Thorne said it showed the force’s “commitment to providing evidence-based answers to the issues which have been raised about this case over many years”.
He added: “This commitment has now resulted in a forensic link between the convicted killer David Morris and an item of great significance which was recovered from the murder scene
“South Wales Police commissioned the review in the hope that we could in some way provide closure for those most affected by the murders.”