She was 15 years old and still charmed by what the area had to offer - especially horses - unlike some other teenagers turned rebellious and promiscuous in those heady 1967 days. Not her.

But on the evening of August 6 she had gone out to meet with some girlfriends as she often did and not returned. Her parents were frantic with worry and went to the police.

With some of the other local lassies known to be a bit wild, the cops would have gone straight to the local spots where secret lovers met. But not Linda Peacock. They knew too she was a good girl.

Parents, police and neighbours searched all night. Early on August 7 in the graveyard of St Mary's Church a lonely bobby hunting his beat found her under a large, thick bush, drizzled with dew and drenched in blood. Dead.

Like any small community, Biggar had had its scandals, its characters, its people falling from grace. But the murder of an innocent 15- year old lassie?

Not on their territory. There was no way the killer would walk free.

Immediately, it seemed as if every soul in the area had volunteered to help search for the killer. The cop in charge was Detective Chief Superintendent William Muncie, top brass of Lanarkshire CID.

The killer wasn't going to walk free on Muncie's shift either.

The forensic bods soon made their awful report. Linda had been bashed on the head with some heavy implement. Then strangled from behind, the burn marks of some rope black on her white neck.

Her clothes were undone, her body exposed but she hadn't had sex - willingly or unwillingly - ever.

Below her body the earth was disturbed and her fingernails were broken and bloody. Wee Linda had struggled and fought every inch of the way. A good girl.

DCS Muncie was on the case and that meant he'd call in whoever could help.

Cops in Lanarkshire needed help.

The Regional Crime Squad from Glasgow was called in and every force in the Central Belt willingly gave troops.

The horror of Linda's murder had grabbed the sympathy of the police as much as it had the public.

As the hunt went on for the killer, Linda was buried in the same graveyard where she'd been murdered. One of the coffin carriers that day was DCS Muncie. This was personal as well as professional. One thing the forensic team had spotted they reckoned would be top evidence - a deep bite mark on her right breast.

They called in Dr Warren Harvey, an expert in forensic odontolgy from Glasgow University. His view was that the bite was unique particularly with both canine teeth having holes or pits. They started examining the teeth of men all over the area.

The Daily Record was on the case sending a reporter up in a plane to take aerial photographs inch by inch. The photos were then passed to the cops. Everyone wanted to help.

From one of these pictures a detective noticed that the Loaningdale Approved School was within very easy walking distance from the scene of crime - as the crow flies.

The school was a residential unit for young men, teenagers sent there by the authorities for offending.

Many residents had a background of violence but mainly gang fights, not targeting women or teenage girls.

But as the most experienced cops knew, a previous pattern of similar violence was often not present in such murders. What they were more hopeful of was in ID - using that bite mark.

The teeth of all the Loaningdale boys were checked. Everyone aside from three of them were quickly eliminated. Then it was down to one - 17-year-old Gordon Hay.

Hay, from Aberdeen, was a thief with no record of violence or having any problem with females.

There was no pattern in his previous behaviour suggesting he was an extreme risk to anyone. But the forensic bods were certain - that bite mark was unique and it belonged to Gordon Hay.

Hay denied murder and had a very strong alibi - on that night he was in bed in his dorm at school.

While Loaningdale wasn't a locked secure unit, it was staffed at all times. As far as the school's records were concerned that was exactly where Gordon Hay was that night - in his bed.

The cops realised this was a good alibi but all hope wasn't lost. They were going to get help from an unexpected quarter.

Loaningdale School was full of career criminals of the future. Young men who already took the stance that they didn't cooperate with the cops.

But like their senior counterparts in the criminal world they didn't like sex killers. They talked.

Many boys slipped out of the school at night to meet local girls who would often wait in the wooded grounds giving owl hoots to draw their attention. Gordon Hay was one of those boys.

Moreover, according to three other Loaningdale boys he had slipped out of school the night Linda was killed.

At his trial on February 26, 1968, at the High Court, Edinburgh, this evidence was spelled out in full. Other local people had also spotted the couple near to the murder spot and one person had heard a girl's scream.

The Crown painted a picture that Hay had slipped out of the school, met Linda and tried to have sex with her. When the young girl refused he lost his temper, battered her head in with a boat hook he'd stolen from another boy and strangled her with a cord.

The cops had part of the cord and found blood on Hay's clothes but they needed more. It all came down to that bite mark.

The defence lawyers knew this and tried to have the presiding judge, Lord Grant, disallow the evidence claiming not that it was unscientific but that the warrant used to take impressions of Gordon Hay's teeth wasn't legal. If they'd won the debate, a killer could have walked free. They lost and the evidence was heard.

In the time-honoured way of courts, expert witnesses argued for both sides. Dr Keith Simpson, pathologist for the Home Office - the top man, in other words - was called as an outside expert. Simpson agreed that the bite was unique.

So did the jury. Gordon Hay was found guilty. He was the first person in the UK to be convicted by forensic dentistry.

That wasn't the end of the issue. The trial had revealed to the public that troubled teenagers could at will walk out of an institution that was meant to have them under control.

If it could happen with Loaningdale in Biggar it could happen with any of the dozen similar schools throughout Scotland.

Demands were made to make all such schools more secure. To keep the young offenders - even if they were legally children - under lock and key.

The management of Loaningdale explained that Gordon Hay had no previous record of violence, never mind extreme violence to women. No one could have predicted that Hay would turn killer.

That didn't reassure Biggar locals even after the national outcry had calmed.

It wasn't good enough that any risk should be run with the innocents of their community. That dangerous young men should be allowed to wander almost freely among them.