It's depressing reading/listening but I recently came across a series on femicides which were never properly investigated and ruled as something else. www.tortoisemedia.com/file/hidden-homicides/
I know there are countless cases of women who aren't taken seriously by the police and then end up dead by their partners, but I'm amazed to learn about how in some cases after the woman has been killed the police don't properly investigate their deaths - despite there being a history of domestic violence.