User blog comment:XGlass Reflection/Ch. 54 Review~ Erwin of the Rebellion/@comment-76.208.146.99-20140220002635/@comment-179280-20140226201230

@ Marco's body was found by his best friend whom he lived three years with. They also incinerated his body. How could Marco be still alive? Hell, he wasn't even important to the story, and even Jean doesn't talk about him anymore. I don't know where's all this freakout about Marco coming from. Perhaps this might be of help if you still think the killer was not Annie and this if you still think Marco is alive.

And about the so called "killer that is totally not Annie". We see this person transform into the Female Titan, and as far as we know, it's not possible for two people to share one Titan form, so it is impossible for this person not to be Annie. She's meant to be drawn differently than usual not to give her identity away.

And about Reiner and Bertolt not moving, well yeah, it's possible that there are more spies since we later have Ymir revealed to also be a Titan Shifter. In this specific case, however, since the person that kills Gunther transforms into the Female Titan, there's no room for doubt.