Let's say there's a ninja (successfully) stealthing and observing a group of orcs. The ninja studies a one of them, uses invisible blade to get greater invisibility, sneaks up to the target, and assassinates one of them the next turn. Now, since assassinate automatically fails if the target recognizes the ninja as an enemy, is she considered "recognized" as an enemy after she makes her first assassination? I ask this because I want to know if she can keep studying and assassinating the other orcs, because they technically didn't see and don't know who exactly the culprit is, even if they know something is out there killing them.