Outside chance theory:
Carbon
dioxide, rather than carbon monoxide?
In Mammoth Lakes, which isn't too far from Hite Cove as the crow flies, there are areas where large amounts of CO2 come out of the ground, due to volcanic activity.
This can kill a human being (and animals) quickly. A 58-year-old man died there from massiv carbon dioxide exposure in 1998.
Here's a 2006 LA Times article about it:
https://www.latimes.com/archives/la-...ks8-story.html
Indeed, when I visited Horseshoe Lake (in Mammoth) some years ago, I saw a sign warning about the carbon dioxide risk.
Maybe there's some sort of geological activity there which they previous didn't know about, and it belched out a bunch of CO2, killing them?
Not likely, but perhaps possible.