I'm sorry for your loss, it's so hard when something like that happens
Are your dogs outside with your chickens often? Often when a dog kills something, it just snaps it's neck and if the dog isn't hungry there is no blood. That's the way my dog kills pests, just breaks their neck or suffocates them, there is no blood.
If it was a wild predator however, there would have been no chicken or a very torn up remains.
I suspect it was your dog, even if they don't act interested with you around, it could just take sudden movement from the chicken to get their prey drive going.
Hopefully other's will tune in as well.

Are your dogs outside with your chickens often? Often when a dog kills something, it just snaps it's neck and if the dog isn't hungry there is no blood. That's the way my dog kills pests, just breaks their neck or suffocates them, there is no blood.
If it was a wild predator however, there would have been no chicken or a very torn up remains.
I suspect it was your dog, even if they don't act interested with you around, it could just take sudden movement from the chicken to get their prey drive going.
Hopefully other's will tune in as well.