Sorry for your loss. If you should lose another chicken in the future with similar symptoms, I would get a necropsy by the state vet or a pcr blood test for Mareks by RAL labs. This link below has a list of state vets to contact about necropsies and costs:
https://www.metzerfarms.com/poultry-labs.html
https://www.metzerfarms.com/poultry-labs.html