If it was in the lungs it was probably mold from something they were fed.
Very high probability.
My first suspicion would be
clover or moldy hay.
Hydrogen Sulfide would be low on my list. Haven't heard of issues in the states, however this was a big problem in Saudi Arabia.
If it was gas leaking from a well they would be able to find it.
Leaks don’t fix themselves so it would still be leaking.