Interesting question. As far as you go, generally speaking, for most crimes, if you had no knowledge of the crime beforehand, no special obligation to act, weren’t involved before or after the crime, etc etc, you don’t have an obligation to report it. Some professions and people have special obligations to report crimes. Teachers for example must report abuse of a child. So it would depend on the circumstances. Here is a good article on the subject that explains accessory before and after as well as some other terms. http://www.ehow.com/about_555857…
The fact that the person is dead adds another component. A dead person cannot be charged for a crime. But anyone who was an accessory before or after, or had some obligation to report, or even got something from the crime, might have an obligation to report it.
Remember, this is for the US.