Footballer Emiliano Sala and his pilot David Ibbotson had been exposed to harmful levels of carbon monoxide in the cockpit of their private plane when it crashed.
An Air Accident Investigation Branch (AAIB) probe found Sala had a carbon monoxide (CO) level of 58 per cent in his blood after the crash on January 21 in the English Channel.
It's believed exposure to that amount of CO can cause seizures, unconsciousness or heart attacks.
It's suspected the pilot , whose body is yet to be found, may have also suffered carbon monoxide poisoning.
The report from AAIB states: "Exposure to CO can lead to damage to the brain, heart and nervous system.
"It is clear from the symptoms that exposure to CO can reduce or inhibit a pilot’s ability to fly an aircraft depending on the level of that exposure."
Sala was flying to Cardiff after he completed a £15million transfer to the former Premier League club.
The AAIB said it was working with the aircraft and engine manufacturers and the National Transportation Safety Board in the US "to identify possible pathways through which CO might enter the cabin of this type of aircraft.
The watchdog added: "Work is also continuing to investigate pertinent operational, technical, organisational and human factors which might have contributed to the accident."
