Sorry for your loss. Can you save the body, keep it wrapped and refrigerated, but not frozen, and send it today to your nearest poultry lab over night shipping? They can do a necropsy, and tell you what is happening in your flock. Here is a link where to contact your state vet:
https://www.metzerfarms.com/PoultryLabs.cfm
https://www.metzerfarms.com/PoultryLabs.cfm