A serial rapist questioned by detectives just two months after Rachel Nickell's death in 1992 and who went on to kill a mother and her young child a year later, has now admitted killing Rachel. But why was he not brought to justice earlier?
Robert Napper slipped through the Metropolitan Police net in three separate manhunts, despite detectives investigating him repeatedly both before and after Rachel's death.
Detectives were even given a tip-off by the killer's own mother, Pauline Lasham, of his confession to a rape on Plumstead Common in November 1989 - but when officers failed to confirm that an offence had taken place nothing was done (it had instead happened nearby in Plumstead itself).
Napper went on to kill Rachel Nickell and 16 months later Samantha Bisset, 27, and her four-year-old daughter Jazmine, as well as carrying out a terrifying series of sex offences throughout south-east London.
Model Rachel, 23, had been walking with her two-year-old son Alex and her dog Molly on Wimbledon Common, south-west London, when she was sexually assaulted and stabbed 49 times in July 1992.
Alex was found clinging to her blood-soaked body, begging: "Get up Mummy."
A year later Napper stabbed Samantha Bissett and smothered her daughter Jazmine, sexually assaulting them both in their Plumstead home.
In October 1995, Napper, who has been diagnosed with paranoid schizophrenia and Asperger's syndrome, was detained indefinitely at Broadmoor high security hospital after admitting the manslaughter of the Bissetts and three other sex attacks on the grounds of diminished responsibility.
Meanwhile, detectives were wrongly pursuing Colin Stagg over Rachel Nickell's murder, believing he matched an offender profile provided by the "real-life Cracker", psychologist Paul Britton.
Napper stabbed Samantha Bissett and smothered her daughter Jazmine
A controversial honey-trap plot with an attractive undercover officer was used to secure a confession from Mr Stagg.
Even when, nine years later, forensic scientists could have traced the killer thanks to advances in technology, it took two reviews and another seven years before a rival forensic laboratory finally made the crucial link.
With only a small boy as a witness to Rachel's death and no forensic evidence, police led by Det Insp Keith Pedder focused on Mr Stagg, who lived nearby and who had also been walking his dog on the common that morning.
Following his arrest, Mr Stagg was fined £200 after admitting indecent exposure on Wimbledon Common for sunbathing nude - but there was insufficient evidence to charge him with Rachel's murder.
Detectives, aided by Mr Britton, then launched the plot using a female detective known as "Lizzie James" to befriend Mr Stagg with suggestive letters, phone calls and dates in Hyde Park during a seven-month "courtship".
The idea was to incriminate or eliminate Mr Stagg from their inquiries.
The 29-year-old continued to deny any involvement in Rachel's death but in August 1993 Mr Stagg was formally charged with her murder, after discussing specifics about her body that detectives believed only the killer could know.
He was held on remand for 13 months before an Old Bailey judge decided the prosecution evidence obtained by Lizzie James was the result of entrapment, questioned Paul Britton's expertise and threw out the case.
Scotland Yard's £3m investigation ended up costing a further £706,000 when compensation was paid to Mr Stagg in August this year.
It took 16 years for the Rachel Nickell case to be solved
Lizzie James, who had suffered from stress and said her career had been ruined by the case, also received £125,000 in an out-of-court settlement in 2001.
A British Psychological Society charge against Mr Britton was dismissed in 2002.
The real killer, Napper, had first come to the police's attention in 1986 when he was given a conditional discharge for carrying a loaded gun.
Following a separate charge in 1992, he was jailed for eight weeks for possessing a firearm.
Between 1989 and 1994, Napper was believed to have been responsible for a spate of up to 106 sex offences in south-east London parkland, which became known as the Green Chain rapes.
One of the attacks was chillingly similar to Rachel's murder.
In May 1992, Napper had tied a ligature around the head of a 22-year-old woman as she pushed her two-year-old daughter in a buggy along King John's Walk, near Mottingham.
He repeatedly beat her head and body, stripped her and raped her. He put a rope around her neck. She begged for her life until eventually he jumped up, dressed and ran off.
Two months later he killed Rachel.
A neighbour alerted detectives to Napper's likeness to the Green Chain rapist's e-fit shortly after, but when he twice failed to give a blood sample it was never pursued, despite another person reporting his likeness to the suspect.
Three days later he was dismissed as a rape suspect (because he was deemed too tall) but he was arrested for possession of a firearm and ammunition without a certificate and sentenced to two months imprisonment in December 1992.
In April 1993, his fingerprints were found on a tin buried in Winns Common - one of the Green Chain rape sites - which contained a handgun, but this was not pursued.
In November that same year, Samantha Bissett was stabbed eight times near her front door and then dragged into the lounge, where Napper mutilated her body.
Samantha's mother Margaret died in 1995
Jazmine was woken up, stripped, sexually assaulted, dressed again, smothered with a pillow and tucked up in bed.
It was only in 1995 that Napper, who lived in Plumstead, was arrested, after a fingerprint that had originally been dismissed as Samantha's was linked to him and also to the Green Chain rapes.
A search of his flat showed drawings and maps of various rape sites.
In October 1995, Napper admitted the manslaughter of Samantha and Jazmine and a rape and two attempted rapes in 1992.
It came too late for Samantha's mother, Margaret, who had died three days earlier at her home near Aberdeen, having never recovered from her daughter and granddaughter's deaths.
Napper's devastated family disowned him long before he was suspected of Rachel's killing.
In a review of cold cases in 2001, the Forensic Science Service (FSS) failed to find any DNA at the Wimbledon Common crime scene.
But in 2004 another forensics company, LGC Forensics, re-examined the case and, by simplifying the over-sensitive procedure the FSS had used incorrectly, they found a DNA match to a formerly unknown male on Rachel's body.
This was later matched to Napper, as well as a fleck of paint from his toolbox found in Alex's hair combings and a footprint found at the scene - meaning there is a one in 1.4 million chance that Napper is not the killer.
Napper has now admitted manslaughter on the grounds of diminished responsibility for the second time and is unlikely ever to be freed.
ROBERT NAPPER'S ATTACKS
1) 1989-1994: String of sex attacks in Mottingham and Plumstead which became known as the Green Chain rapes. One involved a woman who was with her young child.
2) 1992: Murder of Rachel Nickell on Wimbledon Common in front of her two-year-old son.
3) 1993: Murder of Samantha Bissett and her four-year-old daughter Jazmine in Plumstead.