Is there some reason to think they're the only copies?
(*) If he kept the only copy, that's even worse since a new prosecutor wouldn't have access to the old records if they wanted to pursue a new prosecution or if there was an appeal to an old case.
> prosecutor already had access to them
I don't think that argument works. The same could be said for a software engineer. He had access to the source code, so why can't he keep a copy of it?