User blog comment:San&Ashitaka/TITAN THEORIES/@comment-5189653-20131018084454

My theory is that they are all humans, being controlled. When the ape Titan says, So you might know what is in the nape of our necks to Mike Zacarius, I knew that he meant humans. But humans can't kill humans on purpose, so I think they are humans turned evil and controlled by the Ape Titan. I do not know about the Ape Titan though, if it is a human in the nape or not, but all I know is that it is something strong enough to control humans. Also, the reason many humans turn into titans may be because they are being given injections of the type Eren got, so I think there is an evil scientist doing all these bad stuff. And I think he is the Ape Titan at the same time.