Oh! Just thought of another crime that I don't think will ever be technically "solved," although there are a lot of people who KNOW "whodunit!"
Ken Rex McElroy. Town bully of Skidmore, MO. He was known for committing dozens of felonies, including pedophilia, rape, arson, cattle rustling, and burglary; he was generally a horrible person and the last straw was when he shot a local storekeeper with a shotgun (although the man lived). The police would or could do nothing to stop his reign of terror and intimidation, and finally, after he'd threatened to go ahead and kill the man he'd already shot, the town had had enough. On July 10, 1981, Ken Rex McElroy was shot dead in broad daylight, sitting in the cab of his truck with his wife looking on. There were at least two guns used, and perhaps 30 to 40 people who observed the shooting. The entire town has held their silence as to who killed McElroy for almost 30 years now, and there's no indication that they plan to break it now!