The couple jailed for the murder of Margaret Fleming have been urged to reveal what they did with her body.
Police issued a direct appeal to Edward Cairney and Avril Jones on what would have been Margaret's 39th birthday.
The vulnerable teenager was under their care in Inverkip, on the Clyde coast, when she vanished in 1999.
But Jones continued to claim £182,000 in benefits until it finally emerged Margaret was missing in October 2016.
Cairney, 77, and Jones, 59, were ordered to serve a minimum of 14 years after they were convicted at the High Court in Glasgow earlier this year.
Four months on Det Supt Paul Livingstone, the officer who led the investigation, has issued a fresh appeal to the killers.
He said: "I would like to appeal directly to Edward Cairney and Avril Jones, on what would have been Margaret's 39th birthday.
"If you have a shred of decency, you will answer the questions Margaret's family have to allow them to put her to rest."
Det Supt Livingstone has lodged formal requests with Cairney and Jones' lawyers asking for information and has reiterated a previous offer for a face-to-face meeting.
He added: "Margaret was a very vulnerable young woman when she was abused, neglected, manipulated and murdered. It's only right that her family and friends get the opportunity to pay their final respects."
The senior officer also stressed the fact there has been convictions does not mean police would not act on any new information.
Det Supt Livingstone said: " It's very important that she is given the funeral she deserves and for her family to be able to pay their respects to her.
"I would say again to Eddie Cairney and Avril Jones - your lies have caught up with you, so now do the decent thing and let Margaret's family know what has happened to her."
Margaret had been living with the couple for about two years when she disappeared.
During this time detectives said they subjected her to a "living hell".
But despite a painstaking search of their dilapidated property and its garden and an exhaustive proof of life investigation no trace of her has ever been found.
Testimony from Avril's brother, Richard Jones, was used to pinpoint the last independent sighting of the teenager on 17 December, 1999.
Three weeks later, on 5 January, 2000, Avril told her mother, Florence Jones, Margaret had run off with a traveller.
Cairney and Jones, who had no previous convictions, then embarked on a cover up which involved bogus letters and erasing all trace of Margaret from their home.
Police Scotland launched a missing persons' investigation after social work raised the alarm in October 2016.
The couple were both convicted of murder but only Jones was found guilty of benefit fraud as the teenager's money was paid directly into her account.
Despite no evidence to the contrary they maintained Margaret was still alive and often returned to visit them.
Sentencing the pair the judge, Lord Matthews, told them: "Only you two know the truth. Only you know where her remains are."