A forensic review is to be carried out on the evidence which convicted a former builder of the murders of three generations of the same family.
Mandy Power, 34, her bed-ridden mother Doris Dawson, 80, and her daughters Katie, 10, and Emily, eight, were found dead by firefighters at their home in Clydach, near Swansea, in June 1999.
All four had been bludgeoned to death with a pole before their house was set on fire.
Former builder David Morris was jailed for a minimum of 32 years having been convicted for a second time of their murders in 2006.
An earlier conviction was quashed by the Court of Appeal when a second trial was ordered.
The Criminal Cases Review Commission considered Morris’s conviction in 2018 but did not refer his case to the Court of Appeal.
He has long maintained his innocence and last year a programme by BBC Wales Investigates questioned the safety of the conviction.
It featured interviews with two potential witnesses – one who said he had never spoken to police and the other who said he contacted police to report what he had seen but nobody ever called him back.
Since the programme was broadcast, South Wales Police officers have spoken to the two men.
Morris was arrested after the finger of suspicion wrongly pointed to Ms Power’s lover Alison Lewis.
Former policewoman Ms Lewis and her former husband Stephen, an officer with South Wales Police, were arrested on suspicion of murder a year after the deaths.
Mr Lewis’s brother Stuart, also a police officer, was arrested on suspicion of perverting the course of justice.
They were all released without charge.
The Clydach inquiry was the largest and most complex murder investigation ever undertaken by a Welsh police force.
In a statement, South Wales Police said legal representatives of Morris had requested the release of various exhibits from the investigation for further assessment by their forensic scientists.
“This request has been the subject of careful consideration and the force has decided on a proportionate course of action which will involve the appointment of an independent senior investigating officer and an independent forensic scientist to oversee a forensic review of the specific areas referred to by Morris’ legal representatives,” the force said.
“The decision to carry out a forensic review does not constitute a reopening or reinvestigation of the murders, nor does it demonstrate any lack of confidence in the conviction of Morris and the subsequent case reviews.
“Morris was convicted unanimously by a jury on the strength of the prosecution case and independent reviews by the Criminal Cases Review Commission have not identified any new evidence.
“Due to the advancement of forensic technology we may now be in a position to answer some of the questions which have been raised about forensic issues in this case.
“As part of this process, South Wales Police will also be requesting material which has previously been forensically examined by the Criminal Cases Review Commission during its reviews.”