I am sorry for what has happened and I hope you get whatever did this, it is going to take diligence, thats for sure. The predators/dogs are going to come back.
My own dogs killed my 8 yr. old pygmy's last year, I am still sick about it (I no longer have 2 of the dogs)...There were no visible injuries at all, I think the fear/shock/upset just takes its toll and they die. Your Jack is amazing, what a Champ! I wish you all the best, my heart goes out to you.
Tina/tfpets