Uh ... start with the jurisdiction where the case occurred. The local law enforcement agency and maybe the courthouse where the case was heard (if it was prosecuted). Unless the suspect is still languishing in prison, chances are the case file no longer exists, but, you might get lucky. Or, you might be able to find archived news stories about the case and maybe one or more of the investigators might still be alive somewhere.
If you don't even know where it happened, start with names and google them and see if you pick up any leads to help you identify jurisdiction. Keep in mind that cases from even 20 years ago can be hard to locate, so 1 50 year old case will be very difficult and almost certainly none of it will be available electronically unless the various participants were profiled in the past couple of decades or the case hit the news or an appellate decision in the last 20 years or so.