Are you a government official who needs a quick, mundane excuse for a large number of mysterious, possibly supernatural deaths? Gas leaks are the way to go. They're accidental (no need to find a scapegoat!) and provide a good reason to keep people away from the site of the disaster (there might be lingering traces of gas there, after all!). You can even say that the gas was hallucinogenic, so that if any survivors or bystanders saw anything weird, well, that's why.
Worried that anyone with half a brain will see that this explanation doesn't quite add up? Don't be. The general populace will always swallow this one hook, line and sinker, no matter how many times you use it or how implausible it is (maybe it's just that they're more comfortable believing in gas leaks than in demon attacks or the like). Watch out for nosy teenage protagonists, though. Those are a bit harder to fool.