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 wouldn'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 . Her body was found by railway line but her killer has never been caught . Family accusing police of a 'cover-up' after case file was closed until 2040 .