The family of a four-year-old girl who was sexually assaulted and murdered more than 75 years ago have accused police of a 'cover-up ' after officers ordered that the case file be kept secret until 2040 . Joyce Cox was just days away from celebrating her fifth birthday when she was strangled to death on her way home from school in Cardiff on September 28 , 1939 . The schoolgirl had been walking with her older brother Dennis , who was seven at the time , but she disappeared when he lost sight of her . Her body was found dumped by a railway station but police have never caught her killer . Now , Terry Phillips , 71 , who is Joyce 's cousin , has accused the Metropolitan Police of a 'cover-up ' after the force said information could not be released about her murder because it is unfair to the mystery suspect . Joyce Cox was just days away from celebrating her fifth birthday when she was strangled to death on her way home from school in Cardiff on September 28 , 1939 . Her body was found but her killer has never been caught . The front page of the South Wales Echo shows the newspaper report following Joyce 's death in September 1939 , and reveals how officers want to track down a 'man with a slouch ' in connection with her murder . Mr Phillips , who was not born at the time his relative was abducted and killed , used the Freedom of Information Act to find out more information about the case . He contacted the Met about the killing - the oldest unsolved murder in Wales - since the London-based force was drafted in to help South Wales Police with serious crimes at the time . He said that while he managed to establish that the force holds documents about the case , officers slapped an order on the file demanding it be kept secret for 100 years after the events concerned - until January 2040 . A decision letter sent to Mr Phillips said : ' A named subject who was a suspect is described derogatorily and should not be associated with these matters . 'As an unsolved murder , with potential of re-investigation at any indeterminate stage , practice to close for 100 years is invoked . The family of Joyce ( pictured ) are accusing police of a 'cover-up ' for not releasing paperwork on the case . 'However unlikely - indeed remote - it may be that this case is re-opened , we have to afford for that possibility . Putting information into the public domain will include naming specific persons who may yet be identified . 'These persons who may be living may have been witnesses and/or interviewees and who were not prosecuted and who therefore must be regarded as innocent parties . 'Persons will have given witness statements in the expectation that their information would not become public knowledge . ' Mr Phillips said he 'does not accept the reason for refusing to release the documents ' . He said : 'This smacks of a cover-up to me . As a result of this decision , which I am appealing against , I would n't get access to the file until I was 96 , even if I lived that long . 'Joyce 's murder had a devastating impact on the family . Dennis never forgave himself for letting Joyce out of his sight as they walked home . 'He never came to terms with it and died about eight years ago . ' The body of Joyce , who lived in the Whitchurch area of Cardiff , was found on a railway embankment near Coryton station just weeks after Britain declared war on Germany . It was discovered by a dog walker who came across her remains when his spaniel began barking . Dozens of Joyce 's schoolfriends turned out for her funeral on October 4 , 1939 , as documented in the South Wales Echo & Evening Express ( above ) . The funeral procession went through Whitchurch in Cardiff . Joyce was buried at St Mary 's Parish Church in Cardiff and has since been joined by her late mother Irene . Police found a copy of the Western Mail newspaper and a tobacco pouch near her body , and outlined at least two suspects in connection with her murder . Newspaper reports from the South Wales Echo at the time , reveal how police wanted to trace a 'man with a slouch ' in connection with the killing . However , despite interviewing more than one thousand people and following numerous leads , Joyce 's killer has never been caught . Mr Phillips , who is desperate to learn more information about the case , has since been in touch with a leading forensic scientist who has told him that advances in DNA technology could now make it possible to identify the killer . The family believe that had the murder occurred in modern day then her killer would have been caught . MailOnline has contacted Metropolitan Police and South Wales Police for comment .
Joyce Cox was aged four when she was sexually assaulted and murdered . Youngster went missing on her way home from school in Cardiff in 1939 . The case has been reopened after a new DNA test revealed that the killer is not one of his victims.