CCTV holds the key

"He's a cunning individual, violent. He can switch from being nice to being nasty, instantly."
Detective Chief Inspector Colin Sutton, BBC News Online, February 2008
First arrest in connection with the murder of Amelie Delagrange
With growing fears that a serial killer is on the loose, the police are eager for the murder of Bellfield’s final victim Amelie Delagrange to be solved. But they have absolutely nothing to go on. Detective Chief Inspector Colin Sutton leading the investigation decides that clues might be found on CCTV cameras within the vicinity. They track down over 2,000 hours of footage and painstakingly view everything, not knowing exactly what they are looking for. Eventually a white Ford Courier van comes to light. It arrives at Twickenham Green just before Amelie is murdered, and waits for eight minutes before speeding off. The police need to talk to the owner, but they have 26,000 vans to eliminate first. Even help from the DVLA can’t help them and the trail for the killer goes cold.
But a mobile police unit stationed at Twickenham Green has been taking statements from various people in the area. One officer remembers a woman coming in saying that her ex-boyfriend, Levi Bellfield, is a potential suspect. His name is added to the 129 names on the ‘Ex-girlfriends list’. Detectives decide to cross reference this list to see if any own a white van. Bellfield’s name fits the brief. Armed with this information it doesn’t take long to match him to the murder of Marsha McDonnell and the attempted murder of Kate Sheedy. In all three cases Bellfield owned cars that could be placed at each scene. Bellfield is their prime suspect and in March 2006 he’s arrested at his house in West Drayton, London.

Second arrest in connection with the Milly Dowler murder
It’s only after Bellfield is convicted of the murders of Amelie Delagrange and Marsha McDonnell that police start to realise that he may also have murdered Milly Dowler in 2002.
During the course of the investigation officers carry out 3,500 house-to-house inquiries, take 1,850 statements, search 350 sites and trawl through 35 miles of waterways. They discover that at the time of Milly’s disappearance Bellfield lived with his girlfriend Emma Mills at her own home in Collingwood Place, Walton-on-Thames. It’s less than 100 yards from the train station where Milly had last been seen. CCTV also reveals a red Daewoo Nexia was seen in the vicinity the day that Milly disappeared. It’s no coincidence when it’s discovered that Mills owned a car of the same make and model, although to this day, it’s never been found.
During his questioning in July 2005, Bellfield is unable to account for his whereabouts on 21 March 2002. Police also become aware that Bellfield is familiar with Walton-on-Thames and Yateley Heath, where Milly’s body was found. He owns a wheel-clamping business where he’s a frequent visitor to Blackbushe car auctions nearby.
With this compelling evidence Surrey Police submit their report and findings to the Crown Prosecution Service. They in turn decide there is sufficient evidence to charge Bellfield with Milly’s murder. He will once again face a court-room.