One key fact is that Mary Phagan was a young girl who was brutally murdered. Leo Frank was accused of the crime. The case was highly controversial as it involved issues of anti - Semitism as Frank was Jewish. There was a great deal of public outcry and media attention surrounding the trial.
It's not a true story. Usually, such titles are chosen for their intrigue and don't imply a factual basis. Authors often come up with fictional plots to entertain and engage readers.