Leo Frank confessed the murder of Mary Phagan, according to his janitor, Jim Conley, and -- according to the McKnight's, his wife, Lucille Selig.
Based on the statement's of Leo Frank, we know with almost absolute mathematical certainty, Leo Frank either murdered Mary Phagan alone in the metal room at the back of the factory on the second floor, or he enlisted his factory janitor, Jim Conley, to help him.
The pardon of Leo Frank in 1986, was without addressing his guilt or innocence.
Alonzo Mann's testimony in 1982, has been debunked as a hoax ever since then.
I encourage you to read the book, Murder of Mary Phagan, by Mary Phagan-Kean (1988, 1989), surprisingly, it gives the most even handed analysis of the Leo Frank case. You wouldn't expect that to happen, because the author is the grand-niece of Mary Phagan, but the book is the most accurate retelling of case in the last 100 years.
Senator Tom Watson, in his Watson's Magazine issues, August, September and October (1915), gives a legal analysis of the leo frank case from a seasoned criminal defense attorney.