[Mr Ramsbotham]

All three died of respiratory failure and pulmonary edema.

Cyanide, maybe? It's quick, so much so that the victims might not have time to seek help, and I believe I recall reading that it's hard to detect post-mortem.

[Tennessee Nana]

Isnt it cyanide that smells like almonds ???

[gr8eman]

It had to be a concentrated gas that caused this IMO! Sporadic exposure such as an air conditioner leaking refrigerant is unlikely the cause.