First off I'm glad you're lazy.
Second why must the killer always be a man and refereed to as "he", don't women kill too?...
Yep, I'm going with hypnosis. After careful consideration it is the only reasonable explanation to explain how and why Elisa ended up in the tank at the hand of someone else without a fatal wound to her body, since she obviously had to be killed by someone and she couldn't have just had a mental break down and accidentally or purposely killed herself by climbing to the tank.
JMO
I agree with MesaVerde that it's just vastly more likely that a man killed her than a woman... I'll find a better source and more recent info if you want but, according to [ame="http://en.wikipedia.org/wiki/Gender_and_crime"]Gender and crime - Wikipedia, the free encyclopedia[/ame] "In 2004, males were almost 10 times more likely than females to commit murder, including rape-homicides." And the fact that she was found naked suggests to me a sexual assault (though this is by no means established fact yet), which again makes a man a much more likely perpetrator.
As for hypnosis, my understanding is that it's difficult or impossible to hypnotize someone who doesn't want to be hypnotized.
Here is an expert who attests to that. So if she was in an altered state that made her more suggestible or vulnerable, some kind of drugs or mental breakdown would seem more likely.
Also, as MesaVerde points out, the reports that she did not have any signs of trauma on her body could imply suffocation or for that matter poisoning or overdose, right? It doesn't necessarily mean she went up to the roof and into the tank voluntarily (which always seemed to me a highly unlikely scenario). As I wrote before, though, I don't claim any expertise in forensics, so someone more informed is welcome to correct me.
Once again, though, until the toxicology report comes out and the coroner's office renders its final verdict, it's hard to say with any certainty.
PS: Apologies for the redundant Wikipedia link