Emiliano Sala and pilot David Ibbotson were exposed to high levels of carbon monoxide in the plane that crashed into the English Channel in January, according to the Air Accidents Investigation Branch (AAIB) report.
Both men died when the Piper Malibu N264DB plane they were traveling in crashed into the water shortly after departing the French city of Nantes on Jan. 21.
A high saturation level of carbon monoxide was found in Sala's blood when his body was recovered from the wreckage weeks after the incident. Ibbotson's body has still not been found.
"Toxicology tests found that the passenger had a high saturation level of COHb (the combination product of carbon monoxide and haemoglobin)," the AAIB wrote in a statement on its website. "It is considered likely that the pilot would also have been exposed to carbon monoxide."
Sala was on his way to Wales to unite with his new team Cardiff City after the Premier League side made the Argentine its club-record signing.
The 28-year-old's body was repatriated to Argentina and a funeral was held in his hometown of Progreso on Feb. 16.