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 in the English Channel, the Air Accident Investigation Branch said.
Sala, and pilot David Ibbotson, crashed on 21 January when travelling to Cardiff from the French city of Nantes.
Tests on the striker's body found enough evidence of the harmful gas to cause a heart attack, seizure or unconsciousness, an interim report by the Air Accidents Investigation Branch (AAIB) stated.
The cockpit of the Piper Malibu aircraft involved in the crash was not separated from the cabin and it is "likely" that pilot David 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".
Toxicology reports showed Sala's blood showed a carboxyhaemoglobin (COHb) saturation level of 58%.
COHb is the combination product of carbon monoxide (CO) withhaemoglobin, the oxygen-carrying protein molecule contained in red blood cells.
The report said "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's family are calling on the AAIB to salvage the wreckage of the plane "without further delay.”
In response, the AAIB said the decision not to recover the aircraft wreckage has been "explained in detail to both families concerned."
Cardiff City said in a statement they are "concerned" at the AAIB's report.
Piston engine aircraft such as the Piper Malibu involved in the crash produce high levels of carbon monoxide, the 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.
Several devices are available to alert pilots over the presence of carbon monoxide.
The AAIB said they are not mandatory but can "alert pilots or passengers to a potentially deadly threat".
They added that they are working with the aircraft and engine manufacturers and the National Transportation Safety Board (NTSB) in the USA to identify possible pathways through which CO might enter the cabin of this type of aircraft.
It also confirmed work is continuing to investigate "pertinent operational, technical, organisational and human factors which might have contributed to the accident."
A final report will be published in due course.
Argentinian footballer Sala signed for Cardiff City from French club Nantes for £15 million on January 18.
Mr Ibbotson, 59, of Crowle, Lincolnshire flew Sala from Cardiff to Nantes in a Piper Malibu aircraft the following day.
The return flight - which crashed in the Channel - was on January 21.
In June, police said they arrested a 64-year-old man from the North Yorkshire area and has been released from custody under investigation.
In an interim report, the AAIB said that the type of licence held by the pilot meant he could only fly passengers in the European Union on a cost sharing basis, rather than for commercial flights..