Builders renovating a former police station in Nottinghamshire found a bag of evidence from an unsolved murder.
The evidence included bicycle handlebars which the builders said were bloodstained, and is thought to relate to the 1964 murder of Arthur Martin Cope who was beaten to death.
The lost evidence should have been removed from Newark police station when officers moved out seven years ago.
Police said the rusted handlebars were not being treated as the murder weapon.
Mr Cope 56, was bludgeoned in his caravan in Radcliffe-on-Trent.
Baffled police said in a statement: "We are looking into the circumstances as to how this item came to still be in the loft of the building seven years since our departure.
"Forensic tests in the 1960s eliminated the handlebars from the inquiry after no blood was found and, as a result, it was not forensically preserved.
"Over the years they had been exposed to the elements and were rusted."
Police officers previously told the Newark Advertiser it was "embarrassing" for the force to have lost the evidence, which was found in the loft of the former police station.
John Case, now 66, was a police cadet when Mr Cope was killed and later a detective in the Radcliffe area, where the murder happened.
He said the unsolved killing had always been at the back of his mind.
"The inquiry was conducted by senior police officers and the majority, if not all of them, may now have passed away," said Mr Case, who lives in Radcliffe.
"He [Mr Cope] resided in a caravan on a mobile home site in Wharf Lane.
"The caravan was removed for forensic examination and went to our police headquarters, which was then at Epperstone.
"There was a considerable amount of blood in the caravan because I can remember a police officer telling me about that.
"The murder has always been at the back of my mind, as to exactly who did it."
Mr Case compared the cold case to the rape and murder of Nottinghamshire teenager Colette Aram in 1983, which remained unsolved for almost three decades.
Her killer Paul Hutchinson was jailed for life in 2010 after DNA evidence linked him to the crime, and he later killed himself in prison.
Mr Case said he hoped the rediscovery of the handlebars would give detectives another chance to solve Mr Cope's murder, through new techniques such as DNA testing.
Newspaper reports from the 1960s said Mr Cope's injuries were consistent with a "heavy instrument" being used, and a number of potential weapons were recovered from the scene.
The Evening Post, now the Nottingham Post, said the killer appeared to have "gone berserk".
The chief constable of Nottinghamshire Police told the newspaper that nothing was stolen from the caravan and there appeared to be no motive.
"He had tremendous blows on the head and there was blood all over the area in which he was lying," he said.
A team of 60 detectives, including senior officers from Scotland Yard, investigated the murder and more than 4,000 people had their fingerprints taken.
Nottinghamshire Police said processes for the recording and storage of evidence had developed since the 1960s to ensure incidents such as this did not happen.
A spokesperson said further details about the case could not be released while officers worked to trace the victim's family.