It is 15 years today since Hebden Bridge schoolgirl Lindsay Jo Rimer disappeared after leaving home to buy cornflakes.
The 13-year-old's body was found dumped in the Rochdale Canal at Hebden Bridge five months later.
Mystery has surrounded her death ever since.
Despite extensive inquiries, in which more than 5,000 people have been spoken to and several hundred statements taken, police have not been able to find Lindsay's murderer.
Today detectives said they believed someone had information that could lead them to the culprit, and urged them to get in touch.
Detective Supt Colin Prime, from West Yorkshire Police's major investigation review team, is the senior investigating officer in charge of the investigation.
He said: "I firmly believe that someone, somewhere, could hold vital information that would lead us to the killer.
"Although 15 years have passed since Lindsay's disappearance, we continue to seek the vital pieces of information that will bring closure to the investigation and for Lindsay's family."
Lindsay left her home in Cambridge Street, Hebden Bridge, at around 10pm on November 7 1994 to visit the Spar supermarket in Crown Street.
On the way, she called at the Trades Club in Holme Street. According to CCTV footage in the supermarket, Lindsay paid for the cereal at 10.22pm.
Five months later, on Wednesday April 12, 1995, her body was recovered from the Rochdale Canal, about a mile upstream from Hebden Bridge town centre. Detective Supt Prime appealed for anyone with information about Lindsay's murder to get in touch.
He said: "It may be that someone has harboured a suspicion all these years.
"With the passage of time, someone who was unable to come forward and speak to us more than a decade ago might be in a position to do so now. I would strongly urge them to get in touch."
Anyone with any information which could help the inquiry should contact police on 0845 6060606 or CrimeStoppers, anonymously, on 0800 555111.