We called Animal Control this morning when we first found the duck carcass and "crime scene." We were unsure if we should just clean it up or have them aware of it before we did it. When I called they said they were sending out a field agent and to leave it alone until then. They finally came out a couple of minutes ago. Wow, I thought I was weird photographing the carnage, but they did too. They felt it was consistent with a coyote attack and that there have been a dramatic increase in problems with them around here. They felt that our dog scared it off in the middle, then it came back to finish, got scared off again, and then finally gave up after the third time my dog made such a ruckess. The duck was just outside our open front window where it ended up.
However, I am leaning toward it being a stray dog after all the comments here. I guess I won't know for sure.
Now I am feeling it was time and money well spent when I built my coop so secure after reading all the suggestions on strong predator proof construction here at BYC. Thank you for all the advice here and in the Coop Const. forum.