Carbon Monoxide seems possible. They had a rustic home with fireplaces. Architecture Digest did a feature on it. He designed/restored homes as a sort of hobby.
Seems plausible they all contracted an illness which rapidly floored them. The D1.1 strain of H5N1 recently spread to cows there and it isn't like normal Flu and Covid have gone away. Worst flu season in 15+ years in the US. Hackman was 95 and his wife was in her 60's. Doesn't take much at that age.