Please help- death of beloved pet

My husband and I had 3 dogs- I had a male pug, he had a female shepherd mix when we met. When we moved in together they lived together with no problems for two and a half years. We then adopted a six year old female pug together which we had for approx seven months. Three days ago we came home to find the third female pug dead. She had clearly been attacked and bled to death. Obviously we are horribly distraught and are trying to figure out what exactly happened. Obviously neither of us want to believe that it was "our" dog whom could've done such a thing but clearly one (or both possibly) of them did it and I feel that dog may pose a threat to the other dog or other dogs/children/people in the future and may need to be in a one dog/no children household? My question is is it possible for a pug to kill another pug? Are their jaws capable of tearing open another dogs neck? He has never shown any aggression or bit anyone or anything in his four years of life while the collie mix has bit the female pug once, and my parents miniature yorkie once, puncturing his neck. She will also kill birds and small rodents given the opportunity. I'm also wondering if it could've been caused over jealously as my husband had been giving the female pug more attention?? I am at a loss as of what to do. I don't want either of our remaining dogs to get hurt but don't want to assume that the bigger dog did it just because she is bigger. I just can't find any information on pugs attacking anything let alone killing it, via searching the internet. I would truly appreciate any insight or advice you may have on the matter. Our hearts are hurting badly and I just want perhaps a little insight into what could have possibly happened?? Thank you so very much.

I am really sorry for your loss; I can't imagine coming home and seeing that. What makes this really unusual is for about seven months there was no aggression. Usually that happens pretty quickly when a new dog is being introduced to the pack. I take care of a lab and Boston terrier for a friend and her dogs get along with all nine of my dogs and other dogs that are here. About four months ago I gave her another Boston terrier in need of a home and within a couple of weeks the two Boston terriers started fighting. At my place they never did, it only started once the three of them were living together. I think some dogs just won't accept another dog into the pack.
   
From what you described I would bet the Shepard mix killed her, especially with it's past issues with aggression. When dogs go for the throat (like it has before) they are trying to kill the other dog. Please don't take this the wrong way but I would never give someone a dog that I thought was dangerous under any circumstances. Even if there are no children living in the home there is still a chance one might come over. It does sound like this dog's aggression is towards other animals and not people but children are more likely than adults to be bitten.
   
Normally I would say since your first two dogs have gotten along for so long they should be fine together. However waiting seven months to attack the new dog makes me somewhat doubtful. If you seek professional help look for a dog behaviorist who specializes in aggression issues. Most dog trainers (including myself) are not properly trained to handle cases like this. I do have a dog who was pretty aggressive towards dogs and people when I first fostered her about four years ago. I ended up keeping her since I would never adopt her out to someone knowing she may bite. I found the book "How To Right a Dog Gone Wrong" to be very helpful. In a nutshell it tells you how to watch for signs of aggression and what to do. Her aggression is a whole lot better now, but I have to keep an eye on her and watch out for anything that may set her off.
   

   
Again, I'm sorry for your loss and wish you the best of luck.

Thank you so much very much for your kind words and advice :)