In Shawshank Redemption, if Andy actually didn't murder his wife and lover, then who murder them?

Well in the movie, the Shawshank redemption, we see in the beginning of the movie that Andy Dufresne was accused of murdering his wife and his new boyfriend during his trial because his wife divorced him in Reno. But as the film progresses throughout the movie, it tells that Andy is actually innocent that somebody else murders his wife and her lover. But it wanders around my mind, thinking,"Who actually murders his wife and her new boyfriend?"