User blog comment:XGlass Reflection/Ch. 54 Review~ Erwin of the Rebellion/@comment-76.208.146.99-20140220002635/@comment-5189653-20140220010835

It seems it was Annie, as the killer killed gunter, a lightning appeared, then the female titan appeared, and the killer was nowhere to be seen. So it must've been Annie. If not, they might show that in the next few chapters, until then, it is safe to assume that it is Annie because of all the evidence we have on that.