Gardaí and the family of Raonaid Murray have issued an appeal to anyone with information in relation to the murder of the 17-year-old to come forward.
Raonaid was last seen alive in Glenageary in Dublin on the night of 3 September 1999.
She was making her way home from Dún Laoghaire town centre.
Her body was found at Silchester Crescent in Glenageary at 12:33am on 4 September, less than 500 yards from her home.
In their appeal, gardaí said "many who were then Raonaid's age are now parents themselves with children, some of whom would now be close to Raonaid's age, and we would ask them to reflect now with the benefit of maturity and hindsight on any information which may be of assistance to the investigation.
"Despite a large number of suspects having been identified during the course of this investigation, there is in reality no prime suspect.
"The lack of a prime suspect largely arises from the fact that there has been no motive identified in this case. In over 4500 witness statements there is not one word of enmity towards Raonaid."
Gardaí are asking for anyone who has any information which could assist in identifying a motive for the murder and/or anyone who may have doubts about the truth of an alibi already provided, for their immediate assistance.
"You may unknowingly be shielding a killer", the statement said.
The public is being asked to contact the incident room at Dún Laoghaire Garda Station, the Garda Confidential line or any garda station.
Crimestoppers are offering a reward for critical information.
