
13 year-old Mary Phagan was murdered at the National Pencil Company factory she worked on April 26, 1913. She was found early the next morning by the night watchman, a black man named Newt Lee. Mary Phagan had been raped, beaten and eventually strangled. Two notes were found next to her, supposedly written by her as she lay dying, implicating Lee. He was questioned and held with no charges for months.
Two days after Phagan's murder the manager (and part-owner) of the factory, Leo Frank, was charged with murder and jailed. Leo Frank was a Jewish man from Brooklyn, New York, who had moved to Atlanta around 1907 and married in 1910 to Lucille Selig (behind him in the picture). He was charged as much for his heritage as for the fact that he was nervous in his interview with the sheriff. He later testified to that in court:

"Gentlemen, I was nervous. I was completely unstrung. Imagine yourself called from sound slumber in the early hours of the morning ... To see that little girl on the dawn of womanhood so cruelly murdered --- it was a scene that would have melted stone. " - from Wikipedia
The person most consider the true killer was Jim Conley, a black janitor at the factory. He repeatedly admitted to writing the notes; however, he covered it up by saying they were dictated by Frank. His statements changed often and contradicted.
The fit really hit the shan when Frank was convicted due to yellow journalism, circumstantial evidence and a healthy dose of good, old-fashioned bigotry. He appealed his conviction. His appeals to Judge Roan (presiding judge over his trial) and the Georgia Supreme Court were denied. Next they tried the Georgia Supreme Court again, rejected again, and it went to the US Supreme Court. Justice Oliver Wendell Holmes denied his appeal based on a writ of error but did state he felt there was a lack of due process. After one more run at the US Supreme Court, Frank's conviction was upheld (Holmes and Justice Charles Evan Hughes dissenting).
Departing Georgia governor, John Slaton, was Frank's last hope. Frank appealed to the governor for clemency. Slaton reviewed the case carefully and even though he was pressured not to, commuted Frank's death sentence to that of life in prison. He effectively ruined his future political career with that single act of conscience. He simply felt Leo Frank was not guilty and should live until it could be proven.
Frank was moved to a more humane and secure work farm after the commutation. He survived an attempt on his life and was working toward his next appeal when the worst happened. Former governor Joseph Brown and a few others organized the Knights of Mary Phagan. They were virulently anti-Semitic, anti-black and anti-Catholic (sounding familiar?). They planned to kidnap Frank and lynch him. On August 16, 1915, they did just that. They stormed the Milledgeville Prison gates, overpowerd the guards and took Frank to Marietta, Georgia. He was permitted to write a note to his wife and asked that they return his wedding ring to her. Then he was hanged and his body later abused.
None of the lynch mob leaders were ever tried, or even indicted. One of them was actually the prosecuting attorney for the county. In fact, the ringleaders of the Knights of Mary Phagan soon joined the resurrection of the Ku Klux Klan. This trial was the impetus to its recreation, as spurred on by sensationalist journalist Tom Watson (future US Senator). Where the KKK was once only anti-black, now they hated Jews, as well as Catholics and immigrants.
Leo Frank was eventually granted a posthumous pardon - on March 11, 1986 - long overdue.
Crime Library - Leo FrankLeo Frank - WikipediaHistory Channel