my mother in law just told me to tell you she thinks a pack of worf rats. she had a bunch killed by them years ago. looked just as you discribed some birds where carried off others were munched on right there and they show no fear of people or dogs.
if i had to hazzard a guess id say a bird... like a buzzard, which travels with buddys, maybe a small group of juvinile hawks?? just sounds like the usual snach and run of a bird of prey..(the one you found probly fought back and was dropped) and hawks in my area at least attack in the later...