and his wife Betsy Arakawa were feared to have suffered poisoning at their home - and it has earned the eerie nickname of the "stealth assassin" over the years.

Unlike natural gas, carbon monoxide (CO) is colourless, odorless, and tasteless, making it impossible to detect without a specialised device. Inhaling it, from various sources including generators, water heaters, and gas stoves, can prove fatal within minutes.

It was initially thought but the mystery has deepened as further investigation of the home in Santa Fe, New Mexico .

Gene and Betsy, who tied the knot nearly 34 years ago, were . The Oscar winner, aged 95, has been .

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As Gene's daughter said police have not ruled anything out, this morning outlines how CO poisoning has earned the tags of "stealth assassin", "silent killer" and "the invisible enemy". When CO enters the lungs and diffuses into the bloodstream, it replaces oxygen in the blood, preventing it from reaching tissues and organs, including the heart.

The body goes into hypoxia, the state of lacking oxygen. The heart and brain are affected first, as they demand the most oxygen out of every organ and bodily tissue. Without oxygen, brain cells shrivel and die within minutes.

Symptoms can be easily chalked up to something else and dismissed, likely prolonging exposure to the noxious gas. These include an upset stomach, dizziness and confusion.

Shawn Galloway, an occupational safety expert, said: "Some of the initial symptoms are just a slight headache; you could feel a little foggy with a little confusion. You could feel a little weak, but think about just you didn't get enough sleep at night, or you just feel kind of crummy, but it could go all the way to the extreme of seizures, chest pain, the fogginess could lead to difficulty in moving, walking."

CO can seep into the air by gas stoves, currently in use in millions of households, notably across the US. Wood-burning fireplaces or heaters can cause CO poisoning, as can broken water heaters and clogged chimneys. Gas-powered dryers may let off CO.

Mr Galloway told CO poisoning leaves very little lead time to get help when exposure occurs, either intentionally or accidentally. When CO binds to hemoglobin, the protein to red blood cells that carry oxygen from the lungs to the rest of the body, the organs and tissues begin to suffocate.

But the deaths of Gene and Betsy are still being investigated. It could take four to six weeks for an autopsy report to be completed and made public.