Diamond City by Marianna Boncek

In 1955, small-town police chief Art Moran is thrown into a world of danger and mystery when he is tasked with solving a gruesome murder in an isolated mountain cult. Despite feeling out of his depth, he becomes the only person the cult members trust. But things take a turn for the worse when, in a single night, Art loses everything he holds dear, including his love, his son, and even his sanity.