A nine-year-old boy found dead in 1984 after vanishing from his sister's home in Hull where he was babysitting was killed by his uncle, police have said.
The body of Christopher Laverack was found in Beverley Beck two days after his disappearance on 9 March 1984.
Humberside Police have now identified his uncle Melvyn Read, who died in 2008, as the killer. It was the force's longest-running unsolved murder case.
Det Supt Ray Higgins said Read, a convicted sex offender, was "evil".
The schoolboy had been left alone with his 15-month-old nephew Martin at the house in Harpham Grove, east Hull, when his sister Kim Hines left for work and her husband Stephen went out.
Mr Hines returned about an hour later to discover Christopher was no longer there.
The boy's body was found two days later in a plastic carpet underlay bag weighted down by a brick.
Detectives said he had suffered savage injuries and his clothing had been disturbed, suggesting he had been sexually assaulted.
Humberside Police said Read had emerged as a suspect when allegations of child abuse were levelled against him.
In 2003, he was convicted of a number of sex offences involving children and jailed for seven-and-a-half years.
Police said a case had been built up against the uncle, with evidence including the fact that Read owned a car fitting the description of a vehicle seen outside Harpham Grove the night Christopher disappeared.
He was arrested in 2006 in connection with his nephew's death.
But detectives were unable to gather conclusive evidence to charge him with the murder before he died from cancer in Hull prison in February 2008.
Police said a later review of the case had brought a scientific breakthrough.
A forensic palynologist, who specialises in identifying pollen and spores, was brought in to help the investigation.
Dr Patricia Wiltshire concluded there was enough pollen and other plant matter on the clothing Christopher had been wearing on the night to link him with Read's garden at Grantley Grove.
A brick found with the schoolboy's body was also forensically linked to the garden.
In 2012, detectives sought an independent assessment of the strength of their case against Read to see whether there would have been a realistic chance of a conviction in court.
Humberside Police said QC Paul Watson described the weight of evidence against Read as conclusive.
Det Supt Higgins said: "This has been a very complex case to solve but there can be little more important to relentlessly pursue than the murder of a child."
He added: "Read was an evil man and a man wholly without remorse.
"I am satisfied that Christopher's family can have some sense of closure and understanding of what happened."