So sorry that this happened. I had the same thing happen last year. Two of my hens went under the garden gate and came into the back yard. One of my dogs, a beagle/springer mix, ordinarily very gentle, grabbed one of them to play. She rolled her around on the ground and pulled out a lot of feathers. At that moment I looked out and saw her playing with this large black ball, and it took a minute to realize it was a chicken. I rushed out and called the dog. She left the chicken and came right to me, smiling and wagging, not knowing she had done anything wrong. I did not punish her because she did leave the chicken and come to me immediately.
I picked up the hen and looked her over. My brother came out and told me there was another chicken in the yard. I handed him the black one and went to grab the other one. The black hen had no blood, no obvious injuries, just a lot of feathers pulled out. I placed her in the coop on the floor, thinking she might fall off the roost. Unfortunately, we were planning to go out to dinner (my mom was here too), and I left the chickens. We were gone a couple of hours, and when we returned, the hen was dead. I regret that I didn't take her in the house and keep her warm, etc. I might have saved her. I think she died of shock, because she was up and running after the attack.
Why do these things always happen at the most inopportune times??? My brother lives far from me and I don't get to see him very often, so it didn't seem right to back out of the dinner plans. I wish I had. I don't trust either of my dogs around the chickens for an instant.
PS A very effective training tool on a door bolter: I knew a man who had only one leg and was on crutches, and when he tried to take his dog out to hook him to the runner line, the dog would wait till he leaned over to get the snap, and then pull him off balance so he had to let go. The dog would take off. What he did was put a sturdy eye bolt in the door frame INSIDE the door, and hook a nylon line to the dog's collar BEFORE opening the door. Then when they were out on the porch and he was fumbling around and the dog took off, he busted himself about ten feet away when he hit the end of that line going full tilt. He didn't know whether he'd been coming or going. He never bolted again!!!
It can happen in the blink of an eye. I have 4 Catahoula leopard dogs and they are very high prey driven. this will remind me to check and double check for their presence behind me. 
