See all the questions being asked in this thread? That's why they HAVE to do a complete investigation. They don't just come out and say, "Welp, that doesn't make any sense, lock her up". Administrative leave is so bias isn't put into the investigation before it is complete. It's used as an administrative procedure as to not predispose someone as guilty of a crime, therefore tainting whatever comes from the investigation after that.
Now, on to the event. I'm with everyone else. How the heck do you walk in an apartment and not realize it isn't yours?? It'll be interesting what info is pass along after they've finished the investigation. I can't think of a plausible reason she isn't charged and pleads guilty to involuntary manslaughter.