The Air Accident Investigation Branch (AAIB) has said Argentine footballer, Emiliano Sala and his pilot David Ibbotson died from exposure to harmful levels of carbon monoxide in the cockpit of their private plane.
Sala signed for Cardiff City from French club Nantes for £15m on January 18.
Mr Ibbotson, 59, of Crowle, Lincolnshire flew Sala from Cardiff to Nantes in a Piper Malibu aircraft the following day, only to crash in the English Channel on January 21.
The AAIB, in its interim report, stated that tests on the striker’s body found enough evidence of the harmful gas to cause a heart attack, seizure or unconsciousness.
It is likely that Mr Ibbotson was also “affected to some extent” by exposure to carbon monoxide, the document added.
The AAIB said the gas can “reduce or inhibit a pilot’s ability to fly an aircraft depending on the level of that exposure”.
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 report added: “Work is also continuing to investigate pertinent operational, technical, organisational and human factors which might have contributed to the accident.”
A Cardiff spokesperson said: “CCFC is concerned at the AAIB’s latest report which once again highlights that the aircraft used for Emiliano Sala was not appropriate.
“We continue to believe that those who were instrumental in arranging its usage are held to account for this tragedy.”
Piston engine aircraft such as the Piper Malibu involved in the crash produce high levels of carbon monoxide, the interim report said.
The gas is normally conveyed away from the aircraft through the exhaust system but poor sealing or leaks into the heating and ventilation system can enable it to enter the cabin.
The availability of alert for the presence of carbon monoxide, as well as, Mr Ibbotson’s licence will form a key part of their inquiry, according to AAIB.
A full accident report on Sala’s death is expected to be published next year.