Two new clues have been identified in the suspected murder of a teenage girl and her mother 25 years ago.

Police have released details of two cars they are now seeking information about, as they continue to investigate the disappearance of Melissa Trussell, then aged 15, and her mother Rosemary Brown, then 33, from South Australia in 2000.

It comes as detectives have ruled out a car recently found submerged in water near where Brown’s body was found.

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Melissa and her mother were last seen at Blair Athol, in Adelaide’s north, about 2.30am on Saturday, May 13.

Brown’s body was found seven weeks later on July 2, in mangroves at Garden Island in Adelaide’s northwest, but Melissa’s body has never been found.

Police suspect they were both murdered, with the rewards now totalling $1.2 million for anyone who provides information and assistance that leads to the conviction of the person or persons responsible.

A car which was found submerged at Garden Island on July 24 this year has been dismissed as “no longer of interest” in the investigation.

It was found in the search area at Port Adelaide, near where Brown’s body had been found, and examined by Water Operations Unit divers before being ruled out.

However, a “positive” public response to the investigation has since led to new clues, Major Crime Investigation Branch officer-in-charge Detective Superintendent Darren Fielke said.

“As a direct result of information provided by members of the public, two new vehicles of interest have been identified,’’ he said.

“This is a particularly disturbing case. A mother was murdered and her body disposed of and her teenage child remains missing, believed murdered.

“We believe that more than one person was involved in this horrific crime. These offenders murdered a mother and a 15-year-old child.

“Someone knows something about these murders and we implore those who know anything about this case to contact Crime Stoppers.”

The first vehicle is a blue-coloured Ford XD Falcon sedan which a member of the public saw on several nights in the Osborne area in the days following the disappearance of Melissa and her mother, but prior to Brown’s body being found.

The second vehicle is a fawn-coloured Holden VB Commodore sedan with a distinctive brown trim on the lower part of the vehicle.

Police said they would like to again speak to the person who called and provided valuable information in relation to that vehicle, which they said has assisted with progressing the investigation and for which the investigators are “very grateful”.

The caller is urged to contact police again via Crime Stoppers.

Police are also appealing for anyone else who might have information about either of these two vehicles.

This includes anyone who saw the blue Ford Falcon in the Osborne or Garden Island areas between May 13 and July 2, 2000, or who has any information about the fawn Holden VB Commodore.

Fielke said investigations into cold case murders and long-term missing persons were challenging but “there is a dogged determination on the part of the detectives assigned to this investigation to identify and prosecute those responsible for the murder of Rosemary and Melissa”.

“It is vitally important for the family and also to ensure those responsible are held accountable for their actions,’’ he said.

A reward for information and assistance that leads to the conviction of the person or persons responsible for the suspected murder of Melissa was earlier this year increased to $1 million, while there is a $200,000 reward on offer to solve Brown’s murder.

“These rewards are a significant amount of money and can be life-changing for anyone who has information and is willing to come forward and assist,” Fielke said.

“Now is the time to come forward.”

Anyone with any information about the case is asked to contact police.