David Spencer, 13, and Patrick Warren, 11, went missing in the West Midlands on Boxing Day 1996. A decade later, former convicted child murderer Brian Field was interviewed about the deaths.
He has always denied being involved and has not been charged with their disappearance.
This week, David's brother, Lee O'Toole, urged anyone with information to come forward to help bring closure for his family.
Mr O'Toole, 29, said he still suffered nightmares about the day his older brother disappeared.
"I've come to terms with the fact David is dead but I wake every night wondering what happened to him and how he died," he said.
"I just want to find him so he can be laid to rest and we can have somewhere to go and visit him."
The two boys were last seen at a petrol station in Chelmsley Wood, Solihull, near to Field's then home.
Field, 77, who used to work as a milking machine engineer in north Shropshire in the 1980s, was a prolific offender and was twice jailed for offences in the county.
While living in Shropshire he was convicted and fined at Wrexham Magistrates Court or gross indecency, after he passed obscene notes to undercover police in a public lavatory.
In November 1972 he was jailed for two years at Aberdeen Sheriff's Court for indecently assaulting a 14-year-old boy who he pounced on along a country lane.
In April 1982 he was fined for gross indecency at Oswestry Magistrates Court after fondling a 17-year-old boy, then in September 1983 at Shrewsbury Crown Court, Field was jailed for four years after being convicted for two serious sexual offences.
At Stafford Crown Court in June 1986 Field was jailed for four years for falsely imprisoning two teenagers. The boys aged 13 and 16, had been walking between Weston Rhyn and St Martins, near Oswestry, when they were given a lift by Field.
He threatened them with a wheel brace and then forced them to remove their clothing.
The terrified boys escaped by jumping from the moving car.
After his release, Field went to live in the West Midlands where he worked as a farm labourer. Unknown to police he was harbouring a dreadful secret - the murder of 14-year-old Roy Tuthill in Surrey in 1968 as the schoolboy walked home from school. Soon after the murder he moved to Shropshire.
His crime stayed undiscovered until he was arrested for drink driving in 1999 in the Midlands.
A mouth swab showed samples of DNA that matched a sample taken from the scene of the murder of the schoolboy. At first he vehemently denied his involvement, until police told him about the match.
In 2001 he was jailed for life for the murder and is still serving his sentence.
Five years later, officers visited the prison and questioned Field about the disappearance of David Spencer and Patrick Warren. Field had been living near the boys' homes in Rowood Drive, Solihull, working as a gardener and odd-job man when the boys went missing.
Officers reopened the case and dug up a field in nearby Old Damson Lane in a bid to find clues, but abandoned the search several weeks later after finding nothing.
Field denied any involvement in the boys' disappearance. Police said Field was just one line of enquiry in their investigation.
During their investigations police also arrested a 37-year-old man but released him after questioning.
Appealing for new information now, Detective Chief Inspector Caroline Marsh said there were a number of scenarios that could explain the boys' disappearance. "They could have got into an accident and their bodies were never found," she said.
'We are appealing for new information as it's 20 years since their disappearance.
"The family have come to terms with the fact that the boys are probably deceased today.
"Despite all our efforts and all the lines of enquiry, we have not been able to locate the boys. I do believe the boys are deceased. If they were murdered then somebody knows what happened and has some information.
"That is why we are appealing for information and for anyone to come forward."
Last year, Professor David Wilson, of Birmingham City University, famous for the Channel 5 series Killers Behind Bars, said: "If he was responsible, then he is unlikely to ever reveal what happened to them.
"People who have killed repeatedly are often unwilling, silent and uncommunicative.
"They never admit to the crimes they have committed, even when the evidence is in front of them.
"They just won't talk about it. A normal person would want to purge themselves of guilt.
"A psychopath doesn't think like that."
Prof Wilson cited the case of notrious Moors murderer Ian Brady, who, together with Myra Hindley, killed five children and burried them on Saddleworth Moor.
One little boy, 12-year-old Keith Bennet, has never been found and Prof Wilson said that knowledge gave Brady a feeling of power.