I had one killed that way back about mid-Summer. Head was missing and the guts were pulled out but still there (it was obviously interrupted mid-meal). At that time most people guessed or agreed that possibly hawk (and there had been some around). I know this time of year seems to be even worse for hawks. I had to rush mine back in yesterday because a hawk was sitting in the tree above them waiting for me to go inside.
Whatever happened to her, it isn't your fault and I am so sorry. I know how your heart breaks losing one of them.