South Australian Police are re-investigating the cold case disappearance of school girl Rhianna Barreau, who disappeared from her home in Adelaide's southern suburbs almost 20 years ago.

Her relatives have told 7News that in recent weeks they have been approached by Major Crime Detectives on who they think may have taken her.

7News reports that detectives are investigating at least one person who knew Rhianna, but is not related to her.

Rhianna's mother last saw her daughter on Wednesday October 7, 1992,when she left the family home.

Two hours later 12-year-old Rhianna walked to a newsagency in nearby Reynella to buy a Christmas card for her overseas pen pal.

After she bought the card, she was seen walking near Morphett Vale High School, and police said she later returned home.

But once there, it's believed she was lured outside, or left the house later that afternoon, possibly in a rush - relatives said at the time a record was left lying on the lounge room floor, and other items were left out of place.

At the time, police were investigating a Holden Torana with Victorian registration plates that had been reported in the area.

But the Torana was never found and 7News reports that line of inquiry has since been abandoned.

Rhianna's body has never been found, and relatives are hoping recent investigations may offer some hope of a breakthrough.