Caroline Glachan, 14, was attacked and her body found on the banks of the River Leven in West Dunbartonshire on 25 August 1996.
Her mother, Margaret McKeich, believes local people know who is responsible for her daughter’s death and appealed for them to contact police.
She said: “I would urge anybody to come forward, just to give me that peace and to give me closure. There are two questions I need answered – who and why.
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“Take a look back and have a think back and take a look in the mirror. Twenty years has passed – it’s a fair amount of time, but for me that’s just yesterday and I think 20 years is long enough.”
Mrs McKeich, 60, was speaking at a press conference held at Bonhill Community Centre on the Ladyton estate, just yards from where Caroline was last seen.
She was heading to meet her boyfriend in Renton at 11:45pm, having spent the evening with friends, and took a shortcut from shops along Dillichip Loan towards The Towpath and the Black Bridge.
No-one has ever been arrested in connection with her murder.
Mrs McKeich said she is a “wee bit more hopeful than I’ve been for a long time” as police are carrying out fresh DNA tests on hundreds of pieces of evidence.
Detective Superintendent Jim Kerr said DNA testing is “on a different planet” in terms of scientific advances since the items were first tested in 1996 and he is “excited” about the advantages offered.
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He appealed to the community to break the “wall of silence” surrounding the murder and urged a white man wearing a hooded top spotted following the teenager at 12:15am on the day she was murdered to get in contact as he is “probably the last person to see Caroline alive” and has never come forward.
He added: “There are people in this community that know exactly what happened. I feel as if it’s a black cloud over Renton and Bonhill and we need to get it lifted.”
Amanda Pirie, lead forensic scientist with the Scottish Police Authority, said new DNA tests could get results from samples which researchers were previously unable to interpret and could enable samples to be lifted from evidence immersed in water.