The 13-year-old, known as Milly, disappeared while walking home in Walton-on-Thames, Surrey, in March. Her body was found in remote woodland in Yateley Heath Forest near Fleet, Hampshire, on 20 September.
Earlier this year detectives called in the help of the FBI to enhance CCTV footage of a dark sports-style saloon car in Station Avenue, Walton-on-Thames, at the time Milly disappeared.
Police now believe the car is a Mazda MX6, of which only 5,000 have been sold in the UK.
The film, described by police as "hugely significant" at the time, showed a figure that could have been Milly talking to the driver for nearly a minute.
Witnesses
Police are stressing the driver of the car is not necessarily a suspect.
"It is important we trace whoever was in the car as they may be a possible witness," said a police spokeswoman.
Police working on the case have also been trying to trace two men who were spotted with a schoolgirl in the Hampshire woods on the day Milly disappeared.
They are also investigating three cars and a white van seen in the area on the same day.
That sighting was about one hour after Milly went missing.
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