Several people of interest have been identified and a knife has been seized in the investigation into the suspected murder of a mother and her teenage daughter more than 25 years ago.

Rosemary Brown, 33, and her daughter Melissa Trussell, 15, were last seen leaving their South Australian home at Blair Athol about 2.30am on May 13, 2000.

Rosemary's body was located in the Garden Island mangroves weeks later, but Melissa's body was never found.

Items were seized, including a knife that has been submitted for DNA testing to determine whether it has been used in the murders.

"Investigators believe more than one person was involved in the murders and the disposal of Rosemary and Melissa's bodies,'' Detective Superintendent Darren Fielke said.

"To those who played a minor role, or who may h

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