Scott Simpson was nine years old when he was murdered.

His brutal death shocked Scotland and the mistakes made in its aftermath permanently changed the way police handle missing persons investigations.

Scott was playing football in a park a short distance from his home on July 17, 1997, when he was approached by stranger who handed him a note.

"If you want some of the latest Megadrive games... follow me," it said.

Steven Leisk, a serial sex offender with a string of convictions against young boys, lured Scott away from his friends, strangled him with a scarf and dumped his body in the grounds of Aberdeen University.

The schoolboy was not found for five days despite a search involving hundreds of police officers.

Leisk, who had admitted assaulting two 14-year-old boys earlier that year, was only arrested after members of his family called the authorities fearing he had been involved.

The 46-year-old former soldier and Falklands veteran broke down under questioning, weeping as he told officers: "It was an accident, I didn't mean it."

A damning report published in the wake of the murder alleged officers had all the information they needed to solve the case within the first few hours of Scott being reported missing.

It branded Leisk a "violent and predatory paedophile undeterred by arrest and imprisonment".

But social workers who knew Leisk lived in a flat overlooking the park where Scott disappeared did not warn detectives and officers failed to properly interview witnesses.

Scott's body was found in a lane which had already been searched by police.

The revelations led to calls for the resignation of Grampian Police chief constable Ian Oliver, who refused to quit despite being urged to "pack his bags and go".

He was later forced to resign after becoming in embroiled in a sex scandal.

Twenty years after Scott's death, the consequences of the mistakes made continue to affect officers in Aberdeen.

During a recent review of local police, watchdog HMICS said: "It is clear Aberdeen-based officers treat missing person investigations as a 'significant' priority.

"The lessons learnt from a high-profile missing child investigation in 1997 and the priority given to missing persons by legacy Grampian Police remain at the forefront of current officer thinking."

Faster responses to reports of missing people mean that about 10% are found in an hour and more than half are found within 16 hours. Around 99% return home safe.

Police Scotland, which took over from Grampian Police in 2013, handles around 22,000 missing persons cases each year.

Leisk received a 25-year sentence for the murder of Scott Simpson, although this was cut by five years in 2002. He will be eligible for parole later this year.