After sitting through from episode 13-17, two roused my curiosity the most: Erwin and Reiner. Erwin becomes a suspect in the manner he divulged the scout mission in episode 16 in his speech in front of the potential recruits for Recon Corps, about Eren, and then goes on to be more specific about the basement where Eren might discover more secrets. Why is that information pertinent to carry out a high casualty mission? In light of that speech to the new Recon Corps recruits, his prior conversation with the Pixis becomes very intriguing, especially what Pixis had to say to him in episode 15. I now can say without much doubt that Pixis long suspected the demise of humanity is carried out from within humanity's self. There is a mole. More than one, in fact. These group of moles working as a device for some force to carry out humanity's extinction. Titan-Eren's timely entrance finally gives form to Pixis long suspicion as to what these devices are. It's a select group of people that are capable of becoming Titans themselves.
This realization connects the female titan, and most interestingly enough, Reiner. After watching episode 17, I freely admit the identity of the female Titan wasn't as obvious as some people made it out to be at a first glance. I didn't make the connection right away cause to me the clues weren't as obvious. While there might be a few genuine deductive reasoning to reach a certain conclusion, I suspect most of conclusion in the episode 17 discussion thread were made by people who are already spoiled directly (manga reader) or indirectly (by reading threads dedicated to manga, wiki etc.) For me the idea starts to form behind the female titans identity when Armin says, "Jean, avenge my friends who sought to die soon." And those words freezes the female titan midway through a hammer fist directed at Jean. These child soldiers who have already seen their friends die a horrible death in the hands of Titans, those words would tug at their heart string and resonate with anyone that has gone through similar ordeal. Then comes the most important visual clue, the encounter between the female titan and Reiner. What happens after undeniably makes Reiner a suspect and makes female titan's identity clearer. The animators deliberately animates the scene where the female titan squeezes the palm of her hand where she held Reiner. An exaggerated splash of blood comes out in all direction which seemed to signal a horrifying death for Reiner. A second later, Reiner explodes out with no visual damage to himself.
So what exactly is Reiner? is he another titan? If he is, then who is the female titan? Whom did Reiner hung out with during the academy days? And everything
(visual clues such as the hair color, intelligent purposeful eyes, the removal of Armin's hood before running away without killing, etc.) just came together about the female titans identity.
It hasn't been verified yet in the anime, so I'll put the following observation in the spoiler just in case the few remaining genuine anime-only people who still may not have reached the same conclusion, or if this conclusion is just plain wrong (which is I highly doubt
):
Spoiler for please don't click if you don't suspect anyone specific yet:
If this is indeed Annie, why is she working against humanity? What compelled her? Her interaction with Eren in the training ground suggested this is someone who has lead a very hard life already. Her exchange of dialogues with Armin in episode 16 was interesting to say the least..... Following is from gg-subs:
Annie: "You are weak, but you've got guts."
Armin: "T-thank you. Annie, you're actually pretty nice, aren't you? It seems like you don't want us to join the Recon Corps. Do you have reason you want to join the military police?"
Annie: "Not really. It's just.... I want to survive."
In light of episode 17, doesn't the above dialogue suggest Annie is being coerced into playing her role against humanity? When the choices were "survival" or "death", Annie chose survival. She is doing what she needs to survive, but still can't bring herself to kill Armin even in her titan form.
So the bigger question is what forces able to coerce people like Annie and why do they want to make humanity extinct?