Argentine footballer Emiliano Sala and pilot David Ibbotson were exposed to harmful levels of carbon monoxide when their private plane crashed into the English Channel, investigators said.
Sala, 28, was flying from his previous club Nantes in western France to Wales to join Cardiff City when the Piper Malibu aircraft disappeared over the English Channel on January 21.
The wreckage was located following a privately funded search and his body was recovered three days later. The body of Ibbotson has never been found.
Britain's Air Accident Investigation Branch said on Wednesday toxicology tests from Sala's blood showed a carboxyhaemoglobin (COHb) saturation level of 58 per cent. COHb is the combination product of carbon monoxide (CO) with haemoglobin, the oxygen-carrying protein molecule contained in red blood cells.
Levels over 50 per cent lead to seizures, unconciousness and heart attacks, the report warned.
"A COHb level of more than 50 per cent in an otherwise healthy person is generally considered to be potentially fatal," it 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," the bulletin added.
Media reports in the aftermath of the crash had focused on whether the pilot was fit to fly the plane at night.
The BBC said Ibbotson was colour blind, which would have disqualified him from flying during night time.
Principal Inspector of Air Accidents Geraint Herbert warns about the danger of carbon monoxide. pic.twitter.com/e6767DoHKa
— AAIB (@aaibgovuk) August 14, 2019
Sala had been Cardiff's record signing after a fee of £15 million (Dh70m) was agreed with Nantes during the January transfer window.
After travelling to Cardiff to complete the deal, Sala returned to northern France to collect belongings and bid farewell to his teammates.
It was on his return to the Welsh capital to take part in his first training session that the tragedy occurred.
Initial search operations for the player and pilot were suspended in the days after the plane went missing.
However, a crowdfunding effort supported by thousands of donations, including from football stars such as Lionel Messi and Kylian Mbappe, helped launch a private search.
That specialised search saw Sala's body recovered from the wreckage in February but Ibbotson's body has not been found.
Two months after Sala's body was discovered, his father Horacio Sala died of a heart attack in Argentina.
Moving tributes were paid to Sala from across the football world with Nantes retiring the No 9 jersey he wore for the Ligue 1 side.
Sala's family have said the report raises further questions and have called for the wreckage of the plane to be salvaged.
“The family believe that a detailed technical examination of the plane is necessary," lawyer for the Sala family, David Machover of Hickman & Rose solicitors said.
“The family and the public need to know how the Carbon monoxide was able to enter the cabin. Future air safety rests on knowing as much as possible on this issue."
