It was the usual office bet that had to be settled.
In essence, a highly secret state level investigation confirmed their suspicion that the murderers were connected to a robbery ring that operated under the protection of the chief deputy sheriff. Despite learning this, the Governor at the time failed to initiate a state prosecution because it would have impinged on a wealthy local supporter who was a key ally of the sheriff.
So why would two retired cops care about a forgotten case in which the victim was a poor black woman shot at random? Because one of murdered woman's sons had tracked them down and asked who and why his mother had been murdered. There was no gain for the detectives except the moral satisfaction of justice and helping the family of the victim.
Unfortunately, despite the case being an aggravated and racially motivated murder and a failure to prosecute, even in the Obama era, there was no success in the family's suit against the local government. Too much time had passed.