On a cold winter’s day in June 2002, an intellectually disabled teenage girl disappeared from the New South Wales Riverina without a trace.

Since then, the mystery over what happened to Amber Haigh has captivated the vast Australian farming region, due to a stunning allegation: that the 19-year-old was killed by the father of her five-month-old baby and his wife, so that they could take her child.

Two decades on, Robert and Anne Geeves – both 64 – were charged with her murder, but on Monday were acquitted after a high-profile trial.

Justice Julia Lonergan found that prosecutors had failed to prove their alleged motive, saying: “Cases are not decided on rumour, speculation or suspicion.”

The Geeveses are the last known people to have seen Amber alive, and have long said they dropped her at a train station 300km (186 miles) from their home in Kingsvale – where the three had been living at the time – so that she could visit her dying father on 5 June.

Despite extensive police investigations, a coronial inquiry, and a million-dollar reward for information, her body has never been found.

The Crown relied on witness testimony and documentation to support their theory – that the Geeveses had “manipulated” Amber into having Robert’s baby, and then “removed” her “from the equation” when she wouldn’t relinquish custody.

The court heard the couple had an adult son – who had previously dated Amber – but in the early 2000s still “desperately” wanted another child, having endured several miscarriages and a stillbirth.

However, the defence said the allegation they killed Amber to steal her baby was baseless, and that the investigation into the pair – who have spent two years in prison awaiting trial – was flawed from the start.

They told the court a “haze of mistrust” had clouded the local community’s view of the Geeveses due to Robert’s history – which included acquittals for the murder of his ex-partner and a string of sexual offences involving two schoolgirls.

That past, the Geeveses lawyers said, had created a “presumption of guilt” that persisted for decades, and ultimately “blinded” police as they tried to piece together Amber’s case.

Over nine weeks, dozens of witnesses gave evidence about the final months of the teen’s life – describing a “vulnerable” young woman who struggled to discern between “love and exploitation”.

Several recalled how Amber had shared stories of abuse with them – including instances where Robert Geeves had allegedly plied her with alcohol, tied her up, and had sex with her.

And the couple’s son Robbie told the court that his mother had referred to his ex-girlfriend as a “surrogate” and that both parents had turned up at his home in the dead of night to try and force him to accept Amber’s child as “his little brother”.

The prosecution also tendered a written agreement Amber made Robert sign, promising not to take her child, as well as a will she’d created stipulating her aunt be given custody of the baby in the event of her death.

“There was little sign, in the sea of evidence in this case, that Amber was ever shown the love she needed or deserved,” Justice Lonergan said when delivering her verdict.

But she ultimately found that there was a critical “problem” with the prosecution’s case – there was “no satisfactory evidence” that Anne and Robert still held a desire for more children when Amber became pregnant.

She criticised the accounts of prosecution witnesses, and said the investigation had focussed on “disproving the Geeveses version of events” rather than investigating the cause of Amber’s disappearance.

Looking at the couple as they sat in the dock, she ordered that they be released from custody immediately.

As Justice Lonergan delivered the verdict, one member of the public gallery stormed out of the courtroom to scream. Amber’s relatives, too, were visibly shaken, with some later quietly breaking down in tears outside court.