Sheriff Wade was famously corrupt, violent and racist, disappearing one night in the late 1950s at the same time as $10k of town funds went missing.
The general assumption at the time was Sam's own recently deceased father and then Sheriff's deputy, Buddy Deeds (Matthew McConaughey) ran Wade out of town. Buddy took up the role and ushered in an era of relative probity and is considered a local hero, but not by Sam.
CW: racist language
This intergenerational mystery isn't really about the murder mystery. John Sayles is poking at what it takes to be a good person, how love endures, and how things can be seen differently at different times and from different viewpoints.
There are sons who despise fathers and fathers who don't understand their sons. It's race politics, legend and legacy. There's conflict, division and prejudice everywhere. Illegal migrants coming over the border are looked down on by those who did the same just decades ago. The white residents fear being replaced and the tiny number of black residents are resigned to doing their own thing.
Great swathes of it is just 'stuff', whole threads just to show the depth of someone's character that don't touch 'the plot' at all. A really great piece of 90s cinema 9/10 that earns its significant running time.