David ‘Dai’ Morris, 59, was convicted twice of the notorious Clydach murders during which three generations of the same family were wiped out in 1999. Morris – who died at HMP Long Lartin in Worcestershire – always protested his innocence throughout the 22 years he spent in prison and had a large group backing his claims.
A BBC documentary aired last year also cast doubts on the validity of his conviction for the killings which took place in Swansea, south Wales.
This prompted a fresh investigation, but the CPS later ruled there was insufficient evidence to take things any further.
Writing shortly after news of her brother’s death, Debra Thomas said: “I don’t know how to put into words the pain and shock we are feeling right now.
“As you are all aware my brother has passed away. I’m finding this impossible to write.
“Please don’t speculate or listen to rumours, my brother did not die in suspicious circumstances.
“Thank you for all your kind words of support. The fight to prove my brother’s innocence and clear his name will never end.”
The killings shocked the quiet village and made headlines across the country.
Mandy Power, 34, her children Katie, 10, and Emily, eight, were killed along with grandmother Doris Dawson, 80, at their home in Clydach, Swansea.
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Morris was sentenced to four life sentences in 2002 but his conviction was later quashed but he was again found guilty at a retrial.
Campaigners say that Morris was too intoxicated to have committed the murders and have left no DNA evidence at the scene.
A post-mortem revealed that the killer beat Mrs Dawson to death with a heavy fibreglass pole and then lay in wait for Ms Power to return.
When she did, both her and her two daughters suffered the same gruesome fate.
A court heard Morris sexually assaulted and killed Ms Power before inflicting devastating head injuries on her daughters.
To cover their tracks the killer then started fires around the house to try and destroy evidence.
Firefighters and police found the bodies laid out in the hall.
A bloodstained gold necklace was found at the scene which Morris eventually admitted was his.
He claimed it was broken and he’d left it in Ms Power’s house after going around there for a coffee.
The court heard Morris got a new chain and rubbed it in cement and damaged the clasp to make it look like his old one.
Three others were arrested and questioned before Morris was eventually charged with the murders.
A Prison Service spokesman said: “HMP Long Lartin prisoner David Morris died on August 20. The Prisons and Probation Ombudsman has been informed.”
His death will be referred to the coroner.