How did my fish die? Disease or attack

[CJ W]

Hello
I have a 40 gallon tub with a flathead catfish and 2 bluegill, well 1 now because one of them died as recently as last night and a crawfish. However after examining the bluegill it looks as good f my flathead viciously tried to attack and eat my bluegill. What are your thoughts on this?
Here are photos to show the results and a picture to show the size of my flathead

 

[Jmahn]

Maybe both predatory fish seek out the sick/injured by looks of it something has tried to eat it but if that was cause of death is anyone's guess at this point without testing the victim as in looking under microscopes know this won't help much but just my thoughts