That's kinda scary, it might have had rabies. Hope you didn't touch it. Call your county department of health and report it. They will have advice on disposing of it in a safe way and may take it for testing. Or call EnCon (Environmental Conservation).
Our local DEC would respond to something like this. I would call around and see if you can find someone else to dispose of it.
While layer feed does have calcium it can sometimes not be enough if treats are added. To get thing back on track buy some Oyster shell grit and toss a bit on their feed every now and then. This should bring who ever is the culprit back on line. It won't hurt the other hens nor the rooster.
I would be concerned about rabies right off the bat. Don't bury it's carcass, burn it or dispose of it far away from your animals. Do a search on rabies and how to deal with it. Make sure all your animals are vaccinated. Birds don't get rabies as they're not mammals. Or at least that's what I'm told.
Do not touch the carcass with your bare hands. You can take the body to your local vet for rabies testing (as long as the head is intact). Raccoons can die from canine distemper also so hopefully it was just sick from that.
