For that matter, I don't think royal assasinations were something people "covered up". The fact that the usual method of going about them was actually pretty much the same as the Assasins used - ie. one fanatic with a knife or similar usually in a fairly public place where the monarch could be approached relatively easily - pretty much out of necessity as it was unlikely for the killer to get away with it may have something to do with that.

Not that there'd have been much reason for such secrecy either. The next in line and his (or, in some cases, her) supporters would just swear bloody vengeance and divine punishement for such atrocious deed and probably actually make an effort to also realize, even if they were the culprits...