You will need to explore the pages and call them about an autopsy. They do necropsy/autopsy, and testing for many diseases, so look for pathology, autopsy, etc. Here are some links that may help you.
https://www.uoguelph.ca/ahl/contact-us/email-or-phone-us...
Here are some articles about Mareks:
https://www.backyardchickens.com/articles/the-great-big-giant-mareks-disease-faq.66077/
https://vetmedbiosci.colostate.edu/vdl/colorado-avian-health-program/mareks-disease/
Sorry for your loss. That sounds like Mareks disease. Botulism starts with flaccid paralysis of both feet and legs, then progresses to both wings, a limp neck, and eyelids close, followed by death from paralyzed breathing muscles. Mold in feed or scratch, eating moldy compost, or poisoning...