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 .