There has been a breakthrough in the case of Alan Hall, who claims he was wrongfully convicted of the 1985 murder of Auckland man Arthur Easton.
Newshub has learned a group of police officers wrote an anonymous letter to Hall's lawyer claiming a key witness was brainwashed by detectives.
In his decades investigating crime, Tim McKinnel says he's never seen a letter like it.
"It's a revelation," he says.
The letter is entitled 'Some police are honest'. It was sent to Hall's lawyer, raising concerns about his conviction for the murder of Easton.
The offender was originally described by Easton's sons as a 6-foot Māori man, who left behind the murder bayonet and a hat.
"The offender was described as powerfully built, strong like a wild bull," says McKinnel.
But detectives zeroed in on Alan Hall after he said he owned the bayonet and hat left at the scene, items he claimed were stolen from him before the murder.
"Alan Hall, who at the time we know was five foot seven, 68 kilograms," says McKinnel.
Easton's son also said the offender held the bayonet in his right hand.
"Once Alan Hall, who is left-handed, was identified as a suspect that evidence was changed," says McKinnel.
The letter says following a police reconstruction, Easton's son "thought it was logical that the knife was in the intruder's left hand" and "after two to three hours brain washing... now he believes it".
The author also claims detectives "tried to pressure pathologist to say that the intruder was left-handed".
A source wouldn't reveal names to Newshub but said the letter came from a small group of Papakura police officers unhappy with how the investigation was conducted and the accuracy of evidence presented by the prosecution at Hall's trial.
Police wouldn't comment on the letter, but those who headed the investigation always denied its claims.
"To learn that group of police officers were expressing a concern about the integrity of the whole investigation is a very powerful piece of new evidence," says McKinnel.
The investigator now hopes those concerned officers speak to him or to the police.
"There is nothing to fear except the truth emerging," he says.
It's a case destined for appeal this year, as Alan Hall fights to clear his name.