Without little Lee Payne's crucial evidence, his sister's callous killer may never have been convicted.
Still a child himself when his sibling, Sarah Payne, was abducted, sexually assaulted and murdered, Lee caught a glimpse of sick paedophile Roy Whiting as he sped off in his van.
Little did the youngster know that inside was eight-year-old Sarah.
Lee, along with Sarah, his other sister, Charlotte, and brother, Luke, was playing on cornfields in West Sussex when Whiting carried out his vile crime.
The children, along with their parents Sara and Michael, were on holiday at their grandparents house in the village of Kingston Gorse.
It was a glorious summer evening and the youngsters were playing hide and seek when Sarah hit her head and ran off crying.
It was the last time her siblings would ever see her alive.
Sarah was thrown into the back of Whiting's van, which was described at his trial as a "moving prison" and contained a horrifying array of equipment - perfect for the abduction, sex assault and murder of children.
It contained a knife, ropes, baby oil and plastic ties that were looped like handcuffs.
For 48 hours, her family launched a frantic search for missing Sarah. Police and volunteers scoured the local area and soon the hunt went national.
Sarah's parents made desperate TV appeals for information about their daughter and appealed for her safe return.
There were even reported sightings, offering the Payne family some hope their beloved Sarah was still alive.
But then 16 days later Sarah's tiny body was found, naked and dumped in a shallow grave and a murder probe was launched.
Police had already spoken to Whiting in the days immediately after Sarah's abduction.
He was a convicted paedophile who had been jailed for four years in 1995 for the abduction and sexual assault of a nine-year-old girl in Crawley, Sussex.
Whiting was even arrested before Sarah's body was found and held in police custody - but there was no concrete evidence against him and he was released.
Then, on July 23, he stole a car and was chased by police before crashing it and being arrested and jailed for 22 months.
While he was behind bars detectives carried out forensic tests on his van and on September 27, 2000, Whiting was finally charged with the abduction and murder of Sarah Payne.
At his murder trail pathologist Vesna Djurovic told the court Sarah had suffered a ‘violent’ death, probably asphyxia - in a "sexually motivated" attack.
A ball of Sarah's hair provided the DNA evidence police needed to prove Whiting was the one responsible.
It was this ball of hair that was to prove conclusive in the case against Whiting. It contained 200 fibres of material - two which matched the red sweatshirt found in his van.
Eleven came from socks found in the Fiat and one fibre matched the passenger seat front cover. A blue fibre found in the hair matched a fibre in the sweatshirt.
Britain's leading forensic expert, Raymond Chapman, told Whiting's trial that out of 47 hairs found on the sweatshirt, only one provided a DNA profile which matched Sarah.
He added: "The chances of obtaining a match if the hair came from somebody else is in the order of a billion to one."
And Lee also gave the court compelling evidence that placed Whiting at the scene of the crime.
He described how he had seen Whiting’s van pulling away, its wheels spinning and skidding with a screeching sound.
Whiting was grinning and waving at Lee.
The youngster told the court: "He was quite scruffy, looking like he hadn't shaved for ages. He had little white stubbles on his face and little bits of grey in his hair. He was greasy and stuff.
"He had yellowish teeth when he grinned and his eyes were really white and stood out from his face."
This, along with the forensic evidence, was enough to ensure Whiting will spend the rest of his life behind bars.
But despite his role in securing a conviction, ever since his sister was abducted Lee has been wracked with guilt that he could have done more to save her.
He said: "When it comes to feeling guilt about the situation, I did for a few years beat myself up about it.
“Thinking that if I’d maybe run faster, or not run through the corn, I might have caught up with her. Thinking that if I’d had my head on that day, realised she was in the back of that van…”
Since her daughter's death, Sarah's mum, Sara, campaogned tirelessly for a a change in the law allowing people to ask the police if someone with access to their child has a record for child sex offences.
The child sex offender disclosure scheme in England and Wales is also known as “Sarah's Law”.
However, Sarah's dad, Michael, spiralled into depression following his daughter's death.
His and Sara's marriage broke down under the immense strain they were under following Sarah's murder.
Michael suffered from alcoholism and was jailed for 16 months for attacking his brother with a glass in December 2011, during a drunken incident.
In 2014, 45 he was found dead at his home in Kent. He was just 45.