That's a terrible thing to happen JFW67. As Sally said, most of us have had to deal with this. In our case, whilst we have lost several at a time, never all of them because the small flocks were kept separated.
I think it was a mink. We have had a terrier attack and it ate the whole bantam, head, bones and guts included leaving just the wings. It didn't kill all of them either, just the one.
The key here is not how it got in, but how it got out. Overhanging branches are OK for entry, but exit had to be by climbing over the fence. The mesh is quite tightly strung, so climbing is possible. Perhaps the digging was by something else some time earlier?
The coop was rotten, but it's still a small entry point. How did the chickens get outside? were they carried out and an attempt made to drag them over the fence?
Obviously you can't slacken the mesh of the enclosure now (ours is a very rattly loose fit deliberately to prevent climbing) but two strands of electric wire outside will prevent climbing again. It may well have got in via the overhanging branches, which are great for sun shade, but do allow entry for a good climber. Either put something around the trunk or cut the lowest branches back.
Have you found any footprints?