The last time anyone saw Harvey and Jeanette Crewe alive, they were driving home from a stock sale in Bombay about 4pm on June 17, 1970.

It was a cold, wet, winter's day. About an hour later the couple's car was seen again at the south end of their Pukekawa farm, south of Auckland, where Harvey was probably moving sheep from one paddock to another.

No one knows for certain what happened next. Plates on the dining table and a dirty frying pan in the kitchen show their last meal together that day contained peas and flounder.

That evening a neighbour, Julie Priest, heard three rifle shots in quick succession from the direction of the Crewes' farm.

Five days later, after a series of unanswered phone calls to the farm, Jeanette's father, Len Demler, told Owen Priest he had found "a terrible bloody mess" there. The two men went to the house to find bloodstains all over the floor and the couple's 18-month-old grandaughter Rochelle alone in her cot. Priest rang the police, who launched a murder investigation.

The case gripped the country for months. Jeanette's body was recovered from the Waikato River in August. Her jaw was badly broken. Harvey's body followed a month later, weighed down with an axle.

At first the police, led by Detective Inspector Bruce Hutton, considered a murder-suicide but they quickly switched their attention to Len Demler. Despite repeated attempts, they could not draw a confession from Demler or find any evidence that he owned a .22 rifle to match the bullet fragments found in the Crewes' skulls.

After three months the inquiry team was under huge pressure to crack the case. Police were already facing flak over their failure to catch the killers of Welsh hitchhiker Jennifer Beard and Rotorua teenager Olive Walker - both still unsolved murders today.

As journalist Chris Birt records in his book on the Crewe murders, The Final Chapter, an increasingly worried Assistant Commissioner Bob Walton told Hutton at a case conference: "From my point of view, with the Beard one, the Walker one and this one - we have to get one."

Within a week, two crucial pieces of evidence were found, implicating 32-year-old Arthur Allan Thomas, who lived on a farm 13km away. Detective Len Johnston found a stub axle, which apparently matched the axle found tied to Harvey Crewe's body, on Thomas' farm tip.

Police then re-searched the Crewes' garden and discovered a shell case which had been fired from Thomas' .22 rifle.

The farmer admitted to an earlier infatuation with Jeanette Crewe, which police argued was a jealousy motive, possibly linked to a burglary and two arson attacks at the Crewes' in the three years leading up to the murder.

Thomas was charged and convicted of the double murder but the story of the Crewe murders was only beginning. Over the next 10 years his growing army of supporters challenged the police case through the court system, in public meetings and rallies and eventually with their own private investigations (see "The Trials of Arthur Thomas").

As the evidence for Thomas' innocence grew, Prime Minister Robert Muldoon waded into the fray. Thomas was pardoned in 1979 and paid $950,000 compensation a year later. A Royal Commission set up by Muldoon accused Hutton and another police officer, Detective Len Johnston, of "an unspeakable outrage" in planting the shellcase to frame Thomas.

Thirty years later, the case has faded from the headlines but the cloud of suspicion still affects everyone involved. Thomas remains angry that the police never re-opened the investigation or pursued anyone else for the crime - a clear signal that they still regarded him as guilty, despite the pardon. Hutton was never charged with planting evidence but his career was effectively over.

And as a result of the legal stalemate, the three questions which dominated the "Arthur Allan Thomas" case for much of the 1970s are still alive. today.

WHO KILLED THE CREWES?

Most critics of the police investigation believe the murderer was the original suspect, Len Demler, who died in 1992.

His behaviour during the murder investigation was bizarre. According to his own account, when he discovered the bloodstained house, he left his 18-month-old granddaughter alone in her cot and went home, where he cancelled a stock truck before ringing Owen Priest.

After the two men went to the house, Demler returned home and drafted sheep while waiting for the police. He took no part in the search for the bodies of his daughter and son-in-law but often shadowed the searchers on horseback.

Police also knew Demler had a potential motive. He had been fined almost £10,000 for tax evasion in 1961 and forced to sell half the farm to his wealthy wife, Maisie, to pay the bill. When she died, four months before the murders, he expected to get it back but Maisie left her share to Jeanette and Harvey, who reportedly wanted to buy out his father-in-law. Maisie also cut her younger daughter, Heather, out of the will because she had married a divorced American. Birt claims this amounted to a family feud, with Len and Heather on one side, pitched against Jeanette on the other.

The stumbling block for this theory was that Demler apparently did not have a gun which could fire the .22 bullets used in the murder. In Thomas' second trial, police produced an unusual combination rifle-shotgun from Demler to prove this.

Birt argues in his book that witnesses told him Demler's gun had a .22 barrel and could have fired the bullets. But to believe this you have to accept - as many Thomas supporters do - that the rifle produced at the second trial was a fake, with forged registration papers to match.

An alternative theory, originally suggested at the first trial and developed by campaigning journalist Pat Booth, is that the Crewes were arguing over money. Harvey attacked his wife, breaking her jaw, and she shot him in retaliation, probably with his .22 rifle, which was never found. At some stage she contacted her father, who helped her dump the body in the river.

Jeanette then shot herself in despair and Demler got rid of her body, before staging his own discovery of the scene.

The murder-suicide version makes more sense from a psychological point of view. It is easier to imagine a bitter domestic dispute that spilled into murder than a cold-blooded Demler wiping out rival claimants to his farm - let alone a madly jealous Thomas sneaking out at night from the house he shared with his wife and younger cousin.

However, Hutton, who describes the theory as "rubbish", maintains that Jeanette could not have shot herself because the bullet entered her head at at such a steep downwards angle. And Birt says her jaw injuries were so severe she must have died within hours, not days, of receiving them.

Another theory, advanced by Thomas' younger brother Des but rejected by police, is that another man, who still lives in the district, was responsible.

The theory goes back to the original investigation, which found only two rifles in the district could have fired the bullets.

Later a police expert told the Royal Commission that the second rifle could be ruled out, leaving only the Thomas rifle.

Des Thomas maintains the expert was wrong and the barrel of the gun involved was later changed.

A woman in the family that owned the rifle told the Herald in 2006 that this was not true. "I'm very sorry for what happened to them [the Thomas family] but they are grasping at straws."

WHO FED THE BABY?

Two days after the murders, farm labourer Bruce Roddick was feeding out hay from the back of a tractor in a paddock opposite the Crewes' home. He saw a woman, who he assumed was Jeanette Crewe, standing by a car outside the house.

The following day, local woman Queenie McConachie drove past and saw a child who looked like Rochelle in the front paddock.

These sightings have been hotly debated ever since. Combined with doctors' evidence that someone must have fed Rochelle over the five days, they suggest that the woman Roddick saw not only kept the toddler alive - she knew the identity of the killer.

At Thomas' trials, the prosecution claimed it was his wife, Vivien, but Roddick ruled out Vivien at an identity parade and later swore an affadavit it was not her.

Investigative writer David Yallop claimed to have identified the woman in his book, Beyond Reasonable Doubt?, and sent the name in a confidential letter to Muldoon. A Royal Commission the following year said it was not convinced he was right and kept her name confidential.

Birt says the woman identified by Yallop was Heather Souter (nee Demler), the daughter Maisie cut out of her will. But he adds that Yallop was mistaken as Heather was in the United States at the time - a fact confirmed by later police checks.

He claims Roddick actually saw another woman, who still lives in Auckland and who he would not name for legal reasons. In 2006 Birt sent his file on this woman to the police, who said three months later that they had made inquiries but received no new information.

In Booth's version of the murder, Jeanette was the woman seen outside the house. She dressed Rochelle and sent her out to play, put the family car in the garage, moved stock and fed the farm dogs - probably with her father's help.

The stumbling block is that Jeanette was so badly injured, it seems unlikely she could have walked around normally for three or four days. The woman Roddick saw showed no sign of a severely damaged skull.

WHO LEFT THE SHELLCASE IN THE GARDEN?

A used cartridge from Thomas' rifle was found in the Crewes' garden four months after the murder. It was crucial to the case against him.

The Crown argued that Thomas, balanced on a small wall outside the house, shot Harvey Crewe through the open louvre window, hastily reloaded the gun - letting the empty shellcase fall into the garden - and rushed inside to kill Jeanette with a second shot.

Thomas argued from his first trial that someone must have placed the shellcase in the garden but his lawyer Paul Temm was reluctant to raise this, fearing the jury would side with the police instead.

In the aftermath of the second trial, scientist Dr Jim Sprott discovered through an exhaustive analysis that the bullets which killed the Crewes could not have been fired from the Thomas rifle cartridge found in the garden. The two were manufactured at different times and did not match.

In 1980 the Royal Commission into Thomas' conviction, led by outspoken Australian judge Justice Robert Taylor, seized on Sprott's discovery and its implications.

They rejected police claims that they had not searched the garden thoroughly in August, preferring to believe a civilian witness, Graeme Hewson, who said he helped police carry out a comprehensive search.

The implications were important. If Thomas had dropped the shellcase in the garden on the night of the murder, as the Crown claimed, the police would have found it in their detailed August search. The fact that they didn't, the commission reasoned, showed it was not there and must have been planted afterwards.

The commission highlighted evidence from the Crewes' neighbours, Owen and Julie Priest, who recalled hearing shots near the house about a week before the shellcase was discovered.

As the detectives drove away, Owen Priest asked Hutton why they had fired the gun. Priest said Hutton answered, "How do you know?" and he replied, "We heard you".

Based on this exchange, the commission concluded that Hutton and Johnston, who died in 1978, had that day fired the Thomas rifle and planted the shellcase in the garden to frame Thomas for the murders.

However, Hutton was never charged with any offence because a previously secret report to the police by Solicitor-General Paul Neazor found the evidence was too weak.

Neazor's reasons for not prosecuting Hutton - revealed in yesterday's Herald - said any prosecution would have to rely on Hewson's evidence that the Crewes' garden had been thoroughly searched in August, whereas four police officers who carried out the search said the opposite.

He also said the only direct evidence to suggest Hutton or Johnston planted the shellcase came from the Priests, who were unsure when the conversation took place or even whether Johnston was in the car.

He said the conversation amounted to "no more than evidence of opportunity" for Hutton to have buried the shellcase in the garden that day.

This week Hutton greeted the report as a predictable vindication of his innocence.

Former Thomas lawyer Peter Williams replied that the Commission's findings were still the last word and the Crewe murder investigation remained "a classic case of police malpractice".

Forty years on, the battlelines between the two sides remain as firmly drawn as ever.