It's a gruesome mystery. Good to hear you're up out of the reach of snakes. Do you still have the body? If so, can you find a lab to do a necropsy?
If you can't, you can cut her open yourself and examine what you see. It might give you a few clues. Also, if you take photos and post them here...
Maybe a poisonous snake bite? Do you have any snakes around the house and chicken coop? Snake venom could produce these symptoms. If you have a problem with rodents coming around, snakes usually come around, too. I've found rattle snakes in my run on two occasions over the years. Luckily no...
How awful! First, I highly recommend you refrigerate the body of this chick and today call around and locate a lab to do a necropsy. Usually universities have a agricultural lab that will do it.
It sounds like poisoning could be a possibility as cause of death. Were your chicks running around...