Something is just killing my koi. 3rd time it has happened.

pond size 16w x 22L dept 2'to 3'shape kidney. Heavy plant material, plenty of cover to hide under. All have been mornings all have been the orange and white spotted koi, all have been found dead out the water with same injury right at or just after the gill and towards the top of head. The injury is deep at the gill area puncher type then smaller holes at the top the head area. This morn. I found koi # 3 on top of the 20' bridge that goes across the middle of the pond. In the water below was large scales floating....and about 8 feet away from body was large amount of blood????

I live in the city. Could this be birds or something else?

These where not junk koi either!!!!