Best-selling crime writer Denise Mina believes she has solved a 65 year old mystery surrounding one of Scotland's most notorious murders.
The author says she's convinced serial killer Peter Manuel was hired through a third party by Glasgow businessman William Watt to kill his wife in September 1956.
Marion Watt, 45, died along with her 16-year-old daughter Vivienne and Marion's sister Margaret Brown, 41, when Manuel broke into their home in High Burnside, the Daily Record reports.
After being given access to private tapes of the killer talking to a prison psychiatrist for a new documentary on the Crime + Investigation channel, the writer believes Watt paid him to kill Marion, not expecting his daughter and sister-in-law to be in the house.
Denise penned more than a dozen, including The Long Drop about serial murderer Peter Manuel, and 2013 play Driving Manuel, and has a fascination for killer who murdered at least eight people in the 1950s.
Watt, who owned a chain of bakeries, was on a fishing trip in Lochgilphead when they were found shot dead at the family home.
Despite his alibi, police had him down as the prime suspect.
Held at Barlinnie, inmates yelled, "One killer, one scone" as Watt arrived there before being released seven days later.
Denise, 55, said: "He had taken the family's golden Labrador away fly fishing with him. It was supposed to be a guard dog for the house.
"I think Watt asked somebody to kill his wife and they subcontracted it to a dodgy workman who went berserk.
"Marion Watt was pretending she wasn't as ill as she actually was. She'd had open heart surgery.
"She couldn't really manage on her own so her sister came to stay in the house that night and the daughter as well. They were not supposed to be there."
In the episode The Peter Manuel Murders, part of new documentary series Once Upon A True Crime, Denise investigates the 11-hour pub crawl Watt and Manuel then took around Glasgow's dodgy drinking dens and discovers Watt paid the killer £150.
She said: "I believe Peter Manuel's meeting with William Watt, when he taunted him with lurid tales of Watt's wife, daughter and sister-in-law, made Manuel feel invincible and emboldened him to strike again. He did so less than a month later."
Born to Scottish parents in New York in 1927, Manuel had committed a string of sexual attacks by the time he was 16, leading to him serving nine years in Peterhead Prison in Aberdeenshire.
Released in 1953, he attacked Mary McLaughlan, 29, two years later.
She reported the incident to police but Manuel successfully defended himself at Airdrie Sheriff Court, achieving a not proven verdict.
Just 5ft 3in, his first known murder victim was Anne Kneilands in 1956. The 17-year-old machinist was found with her skull smashed. He escaped arrest when his father gave him an alibi. The case was dropped due to insufficient evidence.
In September 1956, Manuel broke into the Watt family home in High Burnside, near Glasgow, killing Marion, Margaret and Vivienne.
In December 1957, 17-year-old Isabelle Cooke was stalked, raped and strangled as she walked to a dance in Uddingston.
Manuel was not initially connected to the teenager's disappearance, but later led officers to the area where he'd buried her body in a nearby field.
The crimes that snared Manuel for the Watt killings were the murders of the Smart family - Peter, 45, Doris, 42, and their son Michael, 10.
The three of them were shot dead at their Uddingston home in the early hours of January 1, 1958.
Dubbed "the beast of Birkenshaw", he was tried for eight murders and convicted of seven at a sensational trial at Glasgow High Court.
Denise is given access to recordings made while Manuel was held at Barlinnie Prison.
On the tapes, made by the prison psychiatrist, the killer protests with almost his dying breath that Watt was not the innocent man he claimed to be.
Denise said: "The prison psychiatrist recorded some of Manuel's stories in the weeks ahead of his execution. One of them is about how William Watt paid someone to shoot his family. I'd never heard Manuel's voice before and it was chilling to listen to it in Barlinnie where it was recorded.
"Neither the psychiatrists nor the prison officers believed a word of his story, but I can tell you quite candidly now that Watt definitely knew who shot his wife and he knew where the gun came from.
"There is no doubt about it in my mind. He had paid someone to shoot his wife."
Manuel was hanged at Barlinnie Prison by Harry Allen on July 11, 1958, aged 31.
Denise said: "Manuel's hangman came for him, bound his wrists and took him to the hanging cell. He was buried in an unmarked grave in the prison grounds.
"It was an ignominious end. It took 26 minutes to walk him across the corridor, hang him, certify that he was dead and put him in the ground.
"Normally in a prison when prisoners were executed, the other prisoners would stop eating their breakfast and put their cutlery down as a mark of respect.
"But Peter Manuel was so despised, they didn't even stop eating."
A few weeks before Manuel's execution, Watt announced his engagement to his young fiancee Lorna Craig.
Denise added: "Here's my theory. William Watt may have paid a gangster to kill his wife and that gangster paid Peter Manuel to carry out the killing.
"What nobody expected was for him to go into the house and find two people who were not supposed to be there and go berserk and kill everybody.
"Not everybody will agree with my theory but the two men who know may have taken the truth to the grave."
