I’m so sorry for your loss. Your state vet at Kissimmee can do a necropsy to look for a cause of death or identify a respiratory illness if you keep the body cold and take or send it in as soon as you can. Here is the list of state vet labs:
https://www.metzerfarms.com/poultry-labs.html
https://www.metzerfarms.com/poultry-labs.html