A senior MP has called for a review into the conviction of the man jailed for the 1999 Clydach murders.
David Morris is serving a life sentence for killing a family of four.
Former shadow chancellor John McDonnell says his conviction should be reviewed by the Criminal Cases Review Commission (CCRC), after a BBC investigation spoke to new witnesses and experts.
South Wales Police said Morris was convicted twice at two trials after an "extensive investigation".
Relatives of the victims also said they had no doubt Morris was responsible.
BBC Wales Investigates spoke to new witnesses and experts who put fresh doubt on the conviction.
Mr McDonnell said the case was "increasingly looking toward a miscarriage of justice".
"The CCRC [Criminal Cases Review Commission] should fulfil its responsibility and immediately review the case in light of this potential huge breakthrough of new information from witnesses."
"Nobody would wish for anyone who is innocent to spend time in prison," Mr McDonnell said.
A conference call to discuss the case has been organised between Mr McDonnell, Morris' defence team and other interested parties.
Mandy Power, her 80-year-old mother Doris and Mandy's children Katie, aged 10, and Emily, aged eight, died at their home in Clydach, Swansea.
They had been beaten to death with a metal pole.
Fires were then started in different parts of the house on the night of Saturday 26 into the morning of Sunday 27 June 1999.
Morris has always maintained his innocence.
In the new programme, BBC Wales Investigates interviewed people not called to give evidence at either of his trials.
It also spoke to experts either involved in the original investigation or who had studied the case.
The Criminal Cases Review Commission declined to comment on the details of the case.
It said it could only consider re-examining a case at the request of a convicted party.
"[David Morris] is welcome to make an application should he feel he has new information that he thinks may have a bearing on the safety of his conviction," a spokesman said.
You can watch BBC Wales Investigates The Clydach Murders: Beyond Reasonable Doubt on the BBC iPlayer