Koi attack--any ideas as to culprit please.

[Philthy]

Few years back I had several good sized goldfish, in a smallish ground level pond, and one day I recall finding remains of one floating with the front half missing. Pond was not netted at that point as I figured the fairly dense planting would provide enough cover. I only ever found the one like that (couple of others disappeared whole courtesy of our friendly neighbourhood heron). A bird couldn't inflict that kind of damage and a cat I don't think is big enough (not without making a more gruesome find for you anyway).
On your's is there any other damage to the rest of the body?

If your fish were at the surface in order to be scooped out could be the oxygen in your pond is low..