Emiliano Sala's plane crashed following his move from Nantes to Cardiff.
Emiliano Sala and pilot David Ibbotson were exposed to high levels of carbon monoxide in the cockpit of the plane they were travelling in when it crashed near the isle of Guernsey in January, the UK's Air Accidents Investigation Branch has said.
The Piper Malibu aircraft the pair were in went missing on Jan. 21 after disappearing from air traffic control's radar while flying over the English Channel on the way to Cardiff, Wales.
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Sala's body was found within the wreckage of the plane, while pilot Ibbotson's body is yet to be recovered. Toxicology results released on Wednesday showed the level of carbon monoxide (COHb level) in Sala's blood was 58%. Anything over 50% in a healthy individual is considered to be potentially fatal.
Daniel Machover of Hickman & Rose solicitors, lawyer for the Sala family, said: "That dangerously high levels of carbon monoxide have been found in Emiliano's body raises questions for the family. How he died will be determined at the inquest in due course.
"The family believe that a detailed technical examination of the plane is necessary."
"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.
"Emiliano's family call on the AAIB to salvage the wreckage of the plane without further delay."
The AAIB report states the symptoms of an individual with a COHb level of above 50% include seizures and unconsciousness, and could potentially cause a heart attack.
Sala had just completed a £15 million transfer to Cardiff City from French Ligue 1 side Nantes and had been due to train with his new teammates the day after the plane went missing.
"Symptoms at low exposure levels [to carbon monoxide] can be drowsiness and dizziness, but as the exposure level increases it can lead to unconsciousness and death," Geraint Herbert, head of air accidents for the AAIB, told the BBC.
"The investigation continues to look into a wide range of areas in relation to this accident, but in particular we are looking at the potential ways in which carbon monoxide can enter the cabin in this type of aircraft."
The AAIB did not confirm that carbon monoxide was the cause of the accident. A final report will be published in due course.
ESPN FC has approached Cardiff City and Nantes for comment.