By late autumn 1995, Drs Lin and Shaun Russell had made the difficult decision to uproot their idyllic life in the Nantlle Valley on the edge of Snowdonia National Park, and move with their two young daughters to Kent.

There were mixed emotions about leaving the mountains for Granary Cottage in the picturesque village of Nonington, a short drive from Shaun's new lecturing job at the University of Kent in nearby Canterbury.

But the girls, Megan, six, and nine-year-old Josie, had settled in well at the village school. For Lin, 45, a geologist and lecturer who had enjoyed a successful career working in Africa for many years, family life was now her priority.

It was, Shaun recalled, "a lovely existence".

It was short lived.

Image caption Shaun Russell with Megan and Josie

Image caption Lin with Josie

Just months after relocating, the family was victim to what Kent Police described as "one of the most horrific crimes ever committed".

What happened just before 16.30 BST Tuesday, 9 July 1996, as the girls, Lin and Lucy the dog, walked the two-or-so miles home from school along Cherry Garden Lane, an unmade track, flanked on one side by a corn field and a small wood on the other, is hard to imagine.

They were accosted by a man, tied up with torn strips of damp towels they had used just minutes before at a swimming gala, made to sit in a copse, blindfolded and bludgeoned with a claw hammer, one by one - Lin, Josie, Megan and the dog.

When they were found eight hours later, it was thought all were dead. Josie was found to have a faint pulse. Remarkably, she survived.

She lives and works as an artist in north Wales, having returned to Gwynedd with her father soon after the attack.

Image caption A car passed the Russells on the unmade track before stopping and attacking them

Image copyright Alamy

A year later the crime remained unsolved.

On the anniversary of the murders a Crimewatch appeal prompted a tip off from a psychologist who worked at a local psychiatric assessment centre. Police arrested and charged 36-year-old Michael Stone from Gillingham.

He was convicted in October 1998.

In the absence of any forensic evidence, the jury believed the main thrust of the prosecution's case - three prison inmates who claimed Stone had confessed.

One of the inmates admitted soon after the trial ended that they had lied and another was discredited. A re-trial was ordered.

But one of the inmates, Damien Daley, then aged 26, held firm to his claim that Stone had confessed to him in grisly detail.

Daley, a self-confessed liar, told the court: "I like to get by in life. I am a crook, that's what crooks do: they beg, borrow and steal to get by in life. But if you were to say to me now are you lying, I would say no, I'm not lying."

Image copyright PA Image caption Michael Stone

The judge's summing up to the retrial jury was unequivocal: "The case stands or falls on the alleged confession of Damian Daley."

In late 2001 Stone was once again found guilty and given three life sentences

Sentencing Stone the judge Mr Justice Poole told him: "There can't be anyone in this country who doesn't understand the horror of these offences."

Stone cried out: "It wasn't me your Honour, I didn't do it!"

Since then Stone has launched and failed in two appeal bids.

Over the years circumstantial evidence has been challenged and doubt cast on some of the prosecution's witnesses by his legal team.

Now, two decades on, they say new evidence, seen by BBC Wales Investigates, brings them closer than ever to proving his innocence.

They claim notorious serial killer Levi Bellfield - currently serving two whole life terms for three murders, including schoolgirl Milly Dowler - has confessed that he murdered Lin and Megan Russell.

And crucially, they allege, he has divulged information only the killer or police would know.

Image copyright PA

Image caption Amelie Delagrange, murder; Kate Sheedy, attempted murder; Milly Dowler, abduction/murder; Marsha McDonnell, murder

Stone's legal team also say they have an eye witness who is convinced she saw Bellfield speeding away from the murder scene.

Details have been passed to the Criminal Cases Review Commission - an independent body which investigates suspected miscarriages of justice - in the hope they will refer the case to the Court of Appeal.

But cell confessions by their very nature are controversial and no one has criticised their use as evidence more than Stone.

It is not without irony then that it is one such confession that Stone bitterly blames for his wrongful conviction and now another he hopes will redeem his name and freedom.

When asked to respond to the new allegations, Kent Police said Michael Stone's protests of innocence have been thoroughly tested by the judicial system.

While some insist Levi Bellfield could well be guilty others are convinced Stone met with the justice he deserves.

So what exactly do we know about the two alleged prison cell confessions and the events that led up to them?

The 'confessions'

Michael Stone: Stone had been arrested in connection with the Russell murders but not yet charged. He had, however, been charged with a separate robbery and burglary and was being held on remand at Canterbury prison.

Stone was being linked to the Russell murders in newspaper reports and after hearing inmates making up stories he insisted he was put in solitary confinement to avoid any fabricated confessions.

Damien Daley, 23, claimed Stone confessed all to him, communicating through a gap between the wall and a heating pipe which linked their cells.

Image copyright Ceri Jackson

"It was like being told a horror story," Daley said at the time.

"He talked about wet towels and someone being disobedient or something, trying to get away but then didn't get far and then it carried on. Something about they didn't have what they wanted. They were paupers or something. He said the dog made more noise than they did."

Stone's lawyers argue that the confession was unreliable - all the information was in the public domain and matched reports in that day's Daily Mirror which had been passed to Daley in his cell.

They also argue that Daley, who was on remand for a GBH and arson charges, has since admitted to others that he lied in order to get the charges dropped. The charges were dropped but due to insufficient evidence, according to the CPS.

Three years ago Daley was found guilty of a drugs-related murder.

This, says Stone and his legal team, along with a history of drug taking and mental health problems, further undermines his credibility.

Levi Bellfield: The man it is claimed Bellfield confessed to wishes to remain anonymous. He too has been convicted of serious offences and was housed in the same high security wing as Bellfield who, he said, had grown to trust and respect him.

This is not the first time Bellfield has been linked to the Russell murders. There has been a war of words between the two convicted killers from behind bars at Durham's Frankland prison where they are both being held, which has been reported in newspapers.

A BBC2 investigation of the Russell murders entitled The Chillenden Murders was broadcast in June this year. A panel of experts was given access to all case files to re-examine the evidence.

They concluded that despite advancements in DNA there was still no forensic link to Stone and it was likely another man was at the scene.

Image copyright PA Image caption Bellfield: Bouncer, drug dealer and wheel clamping contractor

It was this two-part programme which is said to have prompted the alleged confession.

In the minutes leading up to its broadcast Bellfield was reportedly "physically, uncontrollably shaking and put it down to being anxious about watching it".

Following many days of lengthy conversations the unnamed prisoner says he had with Bellfield, he made notes and reported what he had been told to his solicitor, a police officer, and a prison liaison officer.

The prisoner said: "He (Bellfield) said 'I've never told anyone this before…'

"'I killed another child and got away with it… the police were never even close'."

Bellfield is alleged to have told him he had spotted the Russells walking home by chance and he stopped.

He said he approached them with a hammer in his hand and Lin had begged him not to hurt her children. Bellfield said, the prisoner claims, he struck her first and then Josie; the dog was killed followed by Megan.

Image copyright PA Image caption Josie (left) returned to north Wales with her father soon after her release from hospital where she now works as a textile artist

The prisoner said: "I said if I was him I would have been a bit more careful, saying it was risky being so close to the road entrance as anyone passing would see.

"He reassured me he attacked them far enough up the lane that it couldn't have been seen by the road."

But even though he wore gloves, Bellfield was reportedly worried about DNA advances saying "my life in jail would be over if they could prove it was me" and that it would "tear his mother in two".

Image caption Legal team Mark McDonald QC and solicitor Paul Bacon (centre) with Michael Stone's sister Barbara

What makes this alleged confession credible, Stone's legal team insist, is that it appears to contain some detail only very few people would be aware of - such as the police or the killer himself.

"Knowing something in a confession that other people would not know goes to the core of credibility of the confession," Stone's barrister Mark McDonald QC says.

Further to that he says, it is corroborative in that the informer has written contemporaneous notes and immediately informed his solicitor what he had been told.

'Forensic link'

Mr McDonald adds: "We have evidence from his confession that's not in the public domain and which includes a possible forensic link."

For legal reasons we are unable to expand on this further.

It is also claimed Bellfield gave information about Milly Dowler, the schoolgirl he abducted and murdered as she walked home from school in Surrey in 2002.

As well as that it is claimed he had a list of 96 other crimes he has never been tried for - which Bellfield denies - and that he said he had accomplices on several of the attacks.

Bellfield has been contacted by BBC Wales Investigates. He denies murdering the Russells and denies having made the confession.

He claims he has three letters from Stone and has complained about his "persistent attempts" to get him to take responsibility. He also alleges Stone has offered to give him a share of any compensation money he might get for wrongful conviction.

Stone vehemently denies this.

Bellfield added that he had challenged Stone to a lie detector test. Stone has spoken about his reluctance to do this claiming he had been advised his history of psychiatric problems and drug addiction could impact its accuracy.

Alibis

Michael Stone: When arrested he did not appear to have one. When asked where he was, he said: "I can't remember. I can't remember for two reasons. One - I was badly on drugs and two - it was so long ago."

Stone claims he has since pieced events together and was with friends in the Medway town of Strood at the time of the murders.

When he was arrested though, his last confirmed sighting was at his mother's home in Gillingham. This meant he would have had the necessary time to drive the 40 miles to Chillenden.

Levi Bellfield: His former partner Johanna Collings, the mother of one of his 11 children, insists that he was with her throughout the day of the 9 July 1996.

She recently confirmed to BBC Wales Investigates: "It was my birthday and we spent whole day from when we got up to when we went to bed together…

"All the rest of the crimes yeah, 100%, again I think he did them but that one (Russell murders) he didn't."

Image caption Bellfield's ex-partner Johanna Collings

Even so, Stone's legal team say police have never tested the alibi because Bellfield has never been investigated for these murders.

"There are questions marks about her alibi," says Mr McDonald, "as to whether or not she is mistaken... That is for others to decide not for me, I don't know. But it's not black and white."

Image copyright Johanna Collings Image caption Early 1990s: Bellfield with Johanna Collings at Yateley horse show in Hampshire, near to where Milly Dowler's body was found

Modus operandi and motive

Michael Stone: Diagnosed with a violent personality disorder and under psychiatric supervision, he was also a known heroin addict with a criminal history dating back to 1972.

In 1981 he was sentenced to two years for attacking a man with a hammer.

During an interview with BBC Wales Investigates from HM Prison Frankland, Stone claims he had confronted a paedophile at his home and that he reached for a mallet, which did not belong to him but was on a table nearby, as the other man tried to strangle him.

Media playback is unsupported on your device Media caption Stone on previous conviction for hammer attack

"It's nothing like attacking a child or a mother and child... there's no similarity really," Stone insists.

Two years later he was sentenced to four-and-a-half years for stabbing a friend while the victim slept; self-defence, Stone says.

In 1987 he was sentenced to 10 years for two armed robberies.

During his second trial for the Russell murders the court heard that Stone supplemented his income by driving around Kent stealing lawnmowers and other easily disposable goods.

One former friend said he knew the area "like the back of his hand" and had driven around its country roads looking for farm houses to burgle.

While admitting he knew parts of Kent well, Stone denies he knew the country lanes around Chillenden well and says the claim was made by a man who had an axe to grind.

The prosecution pointed to the fact that Stone had been resident at a children's home four miles away from the murder scene.

His sister Barbara, who has long campaigned to clear his name, says they were there for just three weeks and were not allowed out to wander the countryside.

Image caption Stone who had endured a difficult childhood began shoplifting and burglary aged 12

Shortly before the Russell murders Stone is reported to have made threats to kill his probation officer and the officer's family after blaming him for a split with his girlfriend.

"Michael's behaviour was increasingly agitated and voluble and he was not amenable to reasoning," read a statement from Stone's psychiatric nurse.

His motive? Robbery, the police said, to fuel a ferocious drug habit. Josie had later recalled the man asking for money. Lin Russell, who had no money with her, was still wearing her necklace and had a watch in her pocket which, some believe, undermines this theory.

Levi Bellfield: Convicted of three murders between 2002 and 2004 - Milly Dowler, 13, Marsha McDonnell, 19, and Amelie Delagrange, 22, as well as attempting to murder Kate Sheedy, then aged 18. Each of the attacks happened in Bellfield's native south west London.

Known as the 'Bus Stop Killer', he randomly murdered Marsha McDonnell and Amelie Delagrange after they got off a bus and walked home at night. He would launch a so-called 'blitz attack' from behind, striking them repeatedly on the head with a hammer.

Experts have found that motive played little part in Bellfield's attacks which they say gave him a kick simply because he kept on getting away with it.

The types of crime committed by Bellfield are very rare, says retired DCI Colin Sutton, the senior investigating officer who finally brought Bellfield to justice in 2008 and 2011.

"The similarities you've got are a woman in a quiet location, a blitz attack with something heavy and blunt like a hammer," he said.

Image caption DCI Colin Sutton, now retired, whose painstaking work finally brought Bellfield to justice

"For no apparent reason, no previous interaction between them as far as we know. And that in itself, you know, just those features make it an extremely rare crime.

"And because of that there is the natural tendency to look at who else do we know who's committed crimes that have got these very rare features and very rare MO (mode of operating)?

"And, of course, you end up looking at Bellfield."

Media playback is unsupported on your device Media caption Edel Harbison survived a hammer attack - Bellfield denies it was him

Geoffrey Wansall, a journalist and author of a book about Bellfield, says his crimes were random, opportunistic and the only motive appeared to be indulging his deep-seated hatred of young, particularly blonde, women and a fascination with schoolgirls.

But Bellfield's criminality was not confined to hammer attacks.

His former partner revealed that in late 1996 she found a jacket, balaclava and carving knife in a bin liner in the garage of the home they shared. She told how he would stalk an alleyway near a neighbouring train station.

"And that was one of his hunting grounds, he used to go and wait for people to see if he could get someone down there," she says.

Levi Bellfield: Profile of 'heinous' serial killer

Michael Stone refused new appeal over Russell murders

Bellfield, it has been reported, also used his job as a nightclub bouncer to lure young teenage girls into a van kitted out with a mattress where he would drug his countless victims before raping them.

Colin Sutton says Bellfield, at one time a registered police informant, was "a very complex character and you have to understand that he committed various different sorts of crime".

"I'm sure that we've only scratched the surface…," he adds. "His criminality knew no bounds in my view and I think he's probably committed hundreds if not thousands of offences over the years."

When we asked if Bellfield could have been responsible for the Russell murders, Mr Sutton said he would be a good suspect, given the manner of the attacks and that he had links to Kent (through selling drugs and wheel clamping and a family caravan).

But, he stressed, a good suspect only in the absence of other facts - that Kent Police remain convinced Stone is guilty and that Bellfield has an alibi from a former partner who, he says, has no reason to shield a man she gave evidence against in court.

Media playback is unsupported on your device Media caption Bellfield's ex-partner Johanna Collings

The jacket

Stone's lawyers say they have new evidence from an eye witness - an un-named woman, now in her 50s, who was driving a couple of miles from the murder scene around the time of the attack.

In a statement given to Kent Police a few weeks after the murders she describes driving home sometime between 16:15 and 16:50 BST and being alarmed by a car which failed to slow down at a junction. The junction was at the end of a road which led directly to the murder scene.

It was a Ford Sierra or Escort, she said, driven by a man with slightly tanned skin, oval face, aged between 35 and 50.

"The car accelerated harshly with the tyres screeching and I heard the gears crunch as a gear change was made," her statement read.

Image copyright REX/Shutterstock

"He was wearing a brown blouson jacket with a stand up collar which was chunky. This man sat tall in his seat. The top of his head was obscured by the driver's sun visor."

Speaking recently to Stone's legal team she said: "What struck me as unusual was it was a very hot humid day… the guy that was driving had a ski jacket on," she says, "and it was done right up over his mouth, like the big high collars, and he also had the sun visor down. And I thought that's so unusual."

The area a couple of miles from the murder scene in the same direction as the hamlet of Rowling where the bloodied towels were later found dumped.

After making her statement, the women says she was not in contact with Kent Police again.

Then last year, she was watching a documentary about the murder of Milly Dowler.

"A picture came up on the screen and it struck me, stunned me," she says. "So much that I paused the television and took a photo of it on my phone… exactly the collar, type of high collar, coat I described on my police statement that I saw on that day.

"And that was a photograph of Levi Bellfield, I believe. But it was the coat."

There were other eye witness reports on the day of the murder who do not describe the same man. Indeed, some say they say two or three men in the area.

Josie herself, who by that stage had recovered sufficiently to relay some details of what had happened, described a man a bit taller than her father who is 6ft, with yellow, spiky hair.

Stone is 5ft 7ins tall with medium brown hair. Bellfield is 6ft 1ins tall and although naturally dark, he dyed his hair from time to time although it is not clear if he had done so in the summer of 1996.

Another witness helped police put together an e-fit of the man she saw driving away from the murder scene. Josie confirmed it was a good likeness.

That same e-fit helped police to identify Stone as a suspect for the first time. But supporters of Stone say it more closely resembles Bellfield.

Media playback is unsupported on your device Media caption Barbara Stone on e-fit likeness

Josie did not pick Stone out of an ID parade held but she was traumatised and said to have been unable to properly evaluate the men. The eye witness who helped draw up the e-fit said Stone "looked familiar" but could not be certain.

The car

Bellfield is alleged to have said in his confession that he was driving his then girlfriend's car when he killed the Russells.

"I think he said a Ford Sierra," the prisoner claims, "not red though but beige which after the murders was burned out and she had claimed on insurance."

A number of witnesses reported to police they had seen a red/brown or beige Ford Escort or Sierra near to the murder scene.

Michael Stone says he did not own a beige car and drove a white Toyota Tercel at the time of the murders. No microscopic evidence of the murders was ever found in his car.

Image caption Stone's Toyota Tercel

Bellfield, who was known to constantly change his cars to avoid detection by police, did have the use of a beige Ford Sierra Sapphire which belonged to his then partner Johanna Collings.

When asked what happened to the vehicle, Ms Collings told BBC Wales Investigates: "Funnily enough it got stolen and found burned out as so many of his cars or they're never found again."

Bellfield denies borrowing his girlfriend's car.

She confirmed he had claimed on the insurance for the car but says it had "probably been the end of in the March" of 1996 - four months before the Russell murders.

The bootlace

A black bootlace was found on the track not far from the murder scene. It was bloodstained from two of the victims.

During Stone's trial a forensic scientist said marks on Megan's neck suggested a lace had been used as a ligature.

He also said the lace could also have been used as a tourniquet by a drug user. The prosecution used this to link heroin addict Stone to the crime scene.

Hairs were found on Josie's shoes and on Lin's Trousers. Red fibres were also found at the scene as well as on Lin's body - neither was related to the victims nor Stone.

A bloodstained finger print found on a lunchbox belonging to one of the girls was also ruled out as being Stone's.

Tests have been conducted on the strips of towels used to tie up the Russells, found in a bag dumped in a hedgerow a mile-or-so from the murder scene.

Image caption Hairs were found on Josie's shoes

Using DNA voluntarily given by some of Bellfield's relatives, results revealed three component matches with his DNA. This shows merely that he cannot be excluded from the DNA. Stone, on the other hand, can be excluded.

Forensic scientist Dr Georgia Meakin - one of the six experts who re-examined the case as part of the BBC's Chillenden Murders programmes - said: "When you determine the evidential weight of that potential contribution, it's a random match probability of just one in 30 which means if you have 30 people in a room, one of them would be a potential contributor. You can see that evidentially speaking it's not very strong."

Evidence missing

The experts said what might make for stronger evidence is to test the bootlace for a) the presence of heroin and b) DNA from the killer using advances in testing techniques.

But the one piece of evidence which could possibly hold the key of the identity of the murderer is missing.

All that remains of the bootlace it is an empty plastic bag with an exhibit label number.

The forensic laboratory which last examined the lace insists it was returned to Kent Police intact.

Kent Police said they had searched for it and said that the lace had not been lost but had been tested to destruction.

That aside, Stone's barrister Mark McDonald QC is undeterred.

"Given what we know about the lack of evidence… presented to the jury in the actual trial," he says, "this confession is so profound, significant, that it goes to the heart of the conviction of Michael Stone. It's unsafe."

BBC Wales Investigates: 'Confession' of a serial killer Thursday at 20:30 GMT BBC One Wales and BBC iPlayer