War criminals are still prosecuted years after the fact. There was one case just last year where a 93 year old former prison guard from Auschwitz was found guilty of being an accessory to murders committed during the Holocaust. I recall that he didn't express much remorse for his actions, and at least he was found guilty in a court of law, and was sentenced to five years imprisonment for his involvement in those over 170,000 deaths. Not much of a sentence, most likely due to his age and probably the passage of time, but that's still something. There is no statute of limitations on murder in the US.
Of course, I realize there's no statue of limitations on murder. My comment is I just don't think anything will take him down, meaning the evidence there right now will not support an indictment.
As an aside and not connected to this case, did you know that New Mexico had a former 15 yr limit on murder prosecutions?
https://stillunsolved.wordpress.com/tag/united-states/ (Check this story about 2/3 of the way down.)