Premier League footballer Emiliano Sala and his pilot were exposed to high levels of carbon-monoxide inside their private plane minutes before it crashed.
Air accident investigators said the gas levels were enough to cause seizures, unconsciousness and a heart attack.
The light aircraft crashed into the Channel on January 21st.
Argentinian Sala was on his way to join Cardiff City after signing for £15 million from French club Nantes.
Investigators are trying to find how the noxious gas found its way into the plane’s cabin.
The Air Accidents Investigation Branch added: “Work is also continuing to investigate pertinent operational, technical, organisational and human factors which might have contributed to the accident.”
Daniel Machover, lawyer for the Sala family, said: “That dangerously high levels of carbon monoxide have been found in Emiliano’s body raises many questions for the family.
“The family believe that a detailed technical examination of the plane is necessary.