The little girl had been playing with a seven year old boy in an area of trees at Barcroft Community Centre in the Barcroft area of Newry on Friday, June 10, 1994 when she was approached by a stranger, aged in his late teens or early twenties, who asked them to help find his brother.
The pair went with the male and were separated in woodland behind the community centre.
The victim was subjected to a prolonged and violent sexual attack.
Detectives are appealing to members of the public who may have information and have not already contacted police to phone the enquiry team.
They would like to hear from any member of the public who initially came forward in 1994 but has not made contact with the enquiry team since.
Detectives are also keen to identify the couple who met the little boy who had been with the victim and was in a distressed state, leaving the area of woodland on the day of the incident and two young males who had been playing at the Community Centre on the day of the incident.
If you prefer to provide information without giving your details you can contact the independent Crimestoppers charity anonymously on 0800 555 111.