Rosemary Brown, 33, and Melisssa Trussell, 15, were last seen in Blair Athol, about 4km from Adelaide’s CBD, at around 2.30am on May 13, 2000.

Nearly two months later on July 2, Rosemary’s body was discovered in mangroves at Garden Island, a city north-west of Adelaide.

Police believe her daughter Melissa, whose body has never been found, was also murdered.

A public plea for information on the 25th anniversary identified two cars police say are of interest to the investigation.

“As a direct result of information provided by members of the public, two new vehicles of interest have been identified,” Detective Superintendent Darren Fielke said.

The first car of interest is a blue-coloured Ford XD Falcon sedan, which was seen by a member of the public in the Osbourne area following Rosemary’s murder and Brown’s disappearance between May 13 and July 2.

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

Police said a Crime Stoppers caller provided information about the Holden to police which has progressed the investigation.

They have issued a plea for the caller to contact police again.

Detectives are still unable to establish the exact role of this car in the suspected double cold case murder.

Photos of the same make and model for both cars have been shared by the public by SA police.

Fielke asked for anyone who may have information about either of the two cars to contact police through Crime Stoppers.

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

“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.”

Fielke said police are doggedly pursuing this case.

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

A $1 million reward remains on offer for anyone with information which leads to the conviction of those responsible.