Caroline Glachan, 14, was brutally attacked and her body was found on the banks of the River Leven in West Dunbartonshire on August 25 1996.
She had suffered blunt trauma to the head and drowned.
Despite extensive investigations, no-one has ever been arrested in connection with her murder.
Now, two decades on from her death, Police Scotland will be making a fresh appeal for information on the BBC’s Crimewatch programme on Monday night.
A clip released by the broadcaster ahead of the show features an interview with Caroline’s mother, Margaret McKeich, who recalled the day her life stood still.
She told the programme: “I looked out the window and there was a police officer with a police woman and I knew. I just knew. I got this unbelievable pain, which I can still feel, just a pain in my heart. I just knew it was her.
“They came up and they asked about her and the clothing that she had on. I told them and they said that they believed that it was her but they would have to identify her.
“Of course by this time it is my birthday so life didn’t begin at 40. For me, it basically ended.”
Mrs McKeich, 60, recently attended a press conference in which she called on the community to reveal the killer, as police plan fresh DNA tests.
She believes locals know who is responsible for her daughter’s death and appealed for them to contact detectives.
She said: “I would urge anybody to come forward, just to give me that peace and to give me closure.
“There’s two questions I need answered - who and why.”
Caroline, from Bonhill, was heading to meet her boyfriend in Renton, having spent an evening with friends, and took a short cut from shops along Dillichip Loan towards The Towpath and the Black Bridge.
On the Crimewatch show, the teenager’s best friend Joanne Menzies, one of the last people to see Caroline alive, returned to the area where they parted for the final time.
Urging those with information to come forward, Ms Menzies told the programme: “Caroline was only a child, she was only 14.
“These people should now stand up and actually finally be counted as a human being and not hiding a sickening secret.”