The hunt for the killer of Amanda “Milly” Dowler has been running for almost six years and is one of Britain's biggest murder inquiries.

Officers from Surrey police have taken 5,000 witness statements, collated 4,800 exhibits and made 3,500 house-to-house inquiries, most in the streets around Walton-on-Thames railway station where the 13-year-old schoolgirl went missing in March 2002.

On 11 separate occasions police called at a flat on Collingwood Place, Walton, going back time and again until someone answered the door.

The occupant who finally spoke to them had not lived there at the time Milly went missing. The previous resident, a woman, had moved. Neighbours said the woman had never caused any trouble or aroused suspicion and appeared to live alone.

A decision was taken to eliminate the address, and its previous occupant, from the inquiry.

Had a different decision been taken — to trace the woman to her new address — detectives would have been led to the home in West Drayton, Middlesex, of Levi Bellfield.

The woman who lived there with him would have been able to tell them that she had owned a red Daewoo car of the kind caught on CCTV near Walton-on-Thames station on the day Milly went missing. The car was reported stolen four days later.

She could have told them that on the night of Milly's disappearance, Bellfield had got out of bed at West Drayton at 4am and told her he was going “to take care of the dog” at her flat in Walton.

Detectives think this might have been when Bellfield went to dispose of Milly’s body.

The wheel-clamper is now the prime suspect for Milly's murder. The details of a car released by police in a new appeal for information are those of the red Daewoo used by Bellfield and his former girlfriend.

One detective told The Times that he was “nine out of 10”. But had the focus switched to Bellfield six years ago then the murders of Marsha McDonnell and Amelie Delagrange and the attempted murder of Kate Sh-eedy couldhave been prevented.

Sources at Surrey police are defensive about their conduct of Operation Ruby, the Milly Dowler investigation. They have devoted huge amounts of time and resources to it and called on the FBI for help. But they emphasise that there was nothing in 2002 to make Bellfield a suspect. At the time, he had nine previous convictions and there was intelligence that suggested he made silent phone calls to women.

“He was way off the radar, it was not until he was picked up in connection with the Amelie Delagrange murder and we started to learn more about him that he began to be a possible suspect,” said one source.

Another opportunity to catch Bellfield was missed by the Metropolitan Police in Isleworth in May 2004 - three months before he bludgeoned Miss Delagrange to death on Twickenham Green.

He had attacked Miss Sheedy with his car — a white Toyota Previa — knocking her down then driving over her twice to try to kill her. She survived and had valuable information about the car. But an elementary error by police investigating the case allowed Bellfield to escape again.

Officers looking for the Toyota on CCTV cameras in the area, studied footage from the day before the incident occurred. The correct day's film had clear images of Bellfield's vehicle.

The Independent Police Complaints Commission has since reprimanded four officers for their failures in the original inquiry into the attack on Miss Sheedy.

In the end, it was a combination of painstaking detective work and a bizarre stroke of luck that established Bellfield as the perpetrator of the string of attacks and murders.

A police hotline set up after the murder of Miss Delagrange received 129 calls from women who suspected that men they knew might be the killer. One woman named Bellfield, who had violently attacked her during their relationship, as a man capable of killing women. He worked a a wheel-clamper, the caller said.

Simultaneously, officers were studying CCTV pictures from the streets around Twickenham Green on the night of the young Frenchwoman's murder.

They appeared to show that she was followed by a white Ford Courier van but were too blurred to yield the vehicle's number plate or its driver. A smudge on the roof and two shiny, aluminium rear plates were the only things that marked it out from the 26,000 other such vehicles in the UK.

The search for the van produced information about a wheelclamper who had bought such a vehicle for cash locally a few months earlier. The man had left his mobile phone with the vehicle's previous owner.

Then came the stroke of luck. When the phone number was punched into the police intelligence system, it matched with the number of a man who had called the anti-terrorist hotline months before to report suspicions about his Italian neighbour.

The man was Levi Bellfield — the same name given by the woman caller. Surveillance revealed that his white van had similar markings to the one caught on CCTV.

His phone had also rung moments before Miss Delagrange was attacked. Bellfield, whose forensic awareness meant that he normally switched his phone off before attacking his victims, immediately switched it off. But both the call and the act of switching the phone off left traces on the network that placed Bellfield at the murder scene.

When police raided his home to arrest him, they found him cowering in the loft.

As detectives investigated his movements, they pieced together a jigsaw which suggested that Bellfield had killed not only Miss Delagrange but was a serial attacker.

The bridge where Miss Delagrange's mobile phone was dumped was a favourite Thames fishing spot for Bellfield. He had owned a white Toyota of the kind used in the attack on Miss Sheedy.

There was a pattern of stalking victims close to bus stops and attacking as they waited for or stepped off public transport.

Paper trails proved that Bellfield had owned or had access to cars either caught on CCTV or seen near the site of the attacks. Detectives in Surrey were alerted and given access to the growing body of material being gathered by their colleagues in the Met.

Bellfield quickly emerged as a key figure. A trawl back trough CCTV material produced images of a red Daewoo in the area. It emerged that a man in a similar car might have attempted to abduct a girl the day before Milly went missing.

A previous girlfriend of Bellfield's had a daughter who went to primary school with Milly. The schoolgirl had been to her classmate's for tea and was thought to have met Bellfield. Might she have got into his car willingly on the day she went missing?

Milly's body was found in September 2002 in woods at Yateley Heath, Hampshire, an area known to Bellfield who often attended car auctions nearby.

Surrey detectives interviewed Bellfield in July 2005 while he was on remand. After today's convictions they issued a fresh appeal for information and offered a £50,000 reward.

A spokesman said: “We hope people who were previously too afraid to speak are now willing to come forward with new information about Milly's killer.”

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