Levi Bellfield is the prime suspect in one of Britain's most notorious unsolved crimes.
Detectives believe the former wheel clamper may be responsible for the abduction and murder of schoolgirl Amanda Dowler.
The 13-year-old, known as Milly, vanished while walking home from school in Walton-on-Thames, Surrey, on March 21 2002.
Six months later her skeletal remains were found 30 miles away by mushroom pickers on Yateley Heath, Hampshire.
Officers have been painstakingly building a case against Bellfield since he was arrested by the Metropolitan Police in November 2004 over other matters.
Detectives hope the high-profile conclusion of the Old Bailey trial will create the breakthrough they desperately need.
The multimillion-pound Surrey Police investigation, codenamed Operation Ruby, could not be publicly linked to Bellfield while he faced other charges.
But it can be revealed today for the first time that Bellfield was arrested and interviewed over Milly's death three years ago.
Officers believe they have compelling circumstantial evidence that links him to the appalling crime and will soon interview him again.
They have relaunched their appeal for help to trace a red Daewoo Nexia captured on CCTV in Walton-on-Thames minutes after Milly disappeared.
It can now be reported that a car of the same colour and model was owned by Bellfield's then girlfriend Emma Mills.
At the time Ms Mills lived in a rented ground-floor flat in Collingwood Place, a cul-de-sac connected to Station Avenue by another street and a footpath.
A red car also features in another major line of inquiry sparked by what may have been an attempted abduction the previous day.
The family of a 12-year-old schoolgirl told police a man in a red car offered her a lift home in Shepperton, less than three miles from Walton-on-Thames.
The Daewoo, worth no more than £500, was reported stolen several days after Milly disappeared and has never been found. Police fear it was crushed.
Detectives cannot account for Bellfield's movements on the day Milly disappeared. His mobile phone was switched off.
But they know he was familiar with Walton-on-Thames and Milly's final resting place on Yateley Heath as he was a frequent visitor to the nearby Blackbushe car auctions.
The inquiry into her disappearance is one of the largest ever undertaken by Surrey Police.
Six years on, a team of officers led by Detective Chief Inspector Maria Woodall remain dedicated to the investigation.
The last confirmed sighting of Milly was at 4.08pm when a school friend waiting for a bus saw her walking home along Station Avenue.
An hour earlier, after her final lesson, Milly left Heathside School in Weybridge and caught a train to Walton with friends.
Detectives have collected a database with the details of more than 11,000 people, including several suspects. First among them is Levi Bellfield.
Senior officers hope the conclusion of the Old Bailey trial will give them fresh opportunities to find evidence that is more than circumstantial.
First, the end of the trial will allow police to approach court witnesses who were blocked from them because of the judicial process.
Secondly, it may encourage some of Bellfield's formerly loyal criminal associates to finally come forward.
Detectives have released the vehicle registration, N503 GLT, and chassis number, KLATF68V1SB554108, of the Daewoo in a renewed bid to trace it.
It has not been seen since Ms Mills reported it stolen from a Hounslow pub car park several days after Milly disappeared.
The police source said: "We are looking for Levi Bellfield's girlfriend's car and have been for some time.
"We are appealing to people who may have been scared in the past and now, for whatever reason, are not. Or people who had allegiances in the past which may no longer exist.
"We are appealing to Bellfield's criminal associates and we think they may have a change of heart now."
In their hunt for the car, police searched 35 miles of waterways and 40 other water sites including reservoirs and lakes in Surrey and neighbouring counties.
Officers believe it is likely one of Bellfield's many contacts in the motor trade may have arranged for it to be crushed by a scrap dealer.
But it may also have been "recycled" into other vehicles, possibly in a "cut and shut" when two damaged cars are welded together.
The car was captured at the junction of Copenhagen Way and Station Avenue by a CCTV camera on the roof of the Unilever building at 4.33pm.
Forensic experts believed the rear of the car appeared to be lower than normal, suggesting it was carrying a heavy load.
Despite sending the CCTV to the FBI for state-of-the-art analysis, police were unable to identify how many people were in the car or any further details.
The source added: "We are hoping it is somewhere, either a record by a scrap dealer of him bringing it in or it could have been recycled as another car."
Detectives do not know if it is the same car involved in the suspicious stranger incident in Shepperton.
The incident, written off by Surrey Police as a "non-crime", was not initially linked to the Milly investigation.
Two years after she was approached, once the spotlight had switched to Bellfield, the schoolgirl did not pick him out of an identity parade.
Officers conducting house-to-house inquiries in the days after Milly vanished did not get an answer at Ms Mills' home at Collingwood Place.
Senior officers said this did not ring any alarm bells at the time because a woman was the registered tenant. Neighbours did not mention to police that a man sometimes stayed there.
Years later, once the link to Bellfield was found, forensic teams stripped the flat bare, taking away carpets, a sofa and a chair. But they found no link to the missing teenager.
Bellfield was arrested while on remand in July 2005 and questioned by detectives from Operation Ruby.
But he refused to be drawn on his links to Walton-on-Thames or Yateley Heath. During a series of interviews, he stone-walled detectives with replies of "no comment".
Further suspicions were aroused when Bellfield refused to account for his movements on March 21.
Analysis of his phone records revealed he repeatedly switched off his handset, something detectives believe demonstrates he was forensically aware.
Police can use triangulation techniques to track mobile phones as they move between masts and their signal areas.
Despite their suspicions, officers admit the method of Milly's abduction is very different from the offences dealt with by the Metropolitan Police inquiry.
Careful analysis of Milly's bones, scattered by animal activity, revealed no injuries. This is inconsistent with the explosive violence shown in the hammer attacks.
None of Milly's clothing or possessions were recovered from the woodland. Officers found only bones and a small amount of hair.
Police are still trying to trace her Heathside school uniform jacket, Nokia mobile phone and distinctive white purse with an ace of hearts logo on it.
The police source added: "We have not given up on Milly.
"It is a current investigation and we have to keep plugging away until we get the break we want and the family get the justice they deserve."