The Zodiac Killer and D. B. Cooper are notable exceptions. And there are probably a bunch more.
It seems statistically unavoidable that there are always going to be some big cases that go unsolved. It'll probably become less and less common - especially since it seems inevitable that nearly all DNA will eventually be traceable via genetic genealogy databases - but some crimes just won't result in any DNA or other significant evidence being left.
It seems statistically unavoidable that there are always going to be some big cases that go unsolved. It'll probably become less and less common - especially since it seems inevitable that nearly all DNA will eventually be traceable via genetic genealogy databases - but some crimes just won't result in any DNA or other significant evidence being left.