Sorry for your loss. If you can keep the bodies cold, but not frozen, to send themmin for a necropsy by the state vet, they canntest for Mareks, and look for a cause of death. Here is a list of state poultry labs:
https://www.metzerfarms.com/PoultryLabs.cfm
https://www.metzerfarms.com/PoultryLabs.cfm