J. Edgar Hoover, then FBI Director, knew the identity of the attackers at the time but kept that information secret because he wasn't an advocate of the civil rights movement. There were four attackers, and the crime was executed by the KKK.
14 years later, the files containing the attackers' identities were disclosed. Only one of them before this had been successfully prosecuted.
Doug Jones then comes into the story. He was appointed U.S. Attorney for the Northern District of Alabama by former President Bill Clinton.
It was already over 20 years after the attack, and one of the attackers, Herman Frank Cash, was already deceased. But Doug Jones took up the case. Using the evidence that was released from the FBI, Jones prosecuted the remaining attackers - Thomas Edwin Blanton Jr. in 2001 and Bobby Frank Cherry in 2002.
According to the story originally retold on History.com, "Sharony A. Green, a professor of history at the University of Alabama, [said] Jones’ victory is particularly significant given the state’s racial politics: Jones, a man who made his name prosecuting the KKK, beat an opponent who, when asked when American was last 'great,' replied: 'I think it was great at the time when families were united, even though we had slavery.'"