Argentine footballer Emiliano Sala and his pilot were exposed to high levels of carbon monoxide inside the cockpit of their plane before it crashed into the English Channel in January, investigators say.

Sala and David Ibbotson were traveling from Nantes, France, to Cardiff, Wales, when their Piper Malibu aircraft crashed, killing them both.

The 28-year-old had joined the Welsh club for a reported £15 million ($19.3 million) from the French side and was heading to Wales after saying farewell to his former teammates.

Ibbotson's body has still not been found, but toxicology tests on Sala showed harmful levels of carbon monoxide in his blood, which could have caused a seizure, unconsciousness or a heart attack.

The report released by the Air Accidents Investigations Branch (AAIB) said it was likely that Ibbotson would also have been exposed to carbon monoxide.

"Toxicology tests found that the passenger had a high saturation level of COHb (the combination product of carbon monoxide and haemoglobin). It is considered likely that the pilot would also have been exposed to carbon monoxide," reads the report published Wednesday.

"When our investigation has concluded, we will publish a final report."

Blood tests showed Sala had a COHb saturation level of 58%. "A COHb level of 50% or above in an otherwise healthy individual is generally considered to be potentially fatal," the report adds.

A lawyer representing Sala's family called for the plane to be recovered for further examination and said the report "raises many questions for the family."

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Gas colorless and odorless

Carbon monoxide, a colorless, odorless gas, is produced in high concentrations in piston-engined planes, but is removed through the exhaust system.

Poor sealing of the cabin or leaks into the heating and ventilation system from the exhaust could allow the gas into a plane, according to the report.

"Whilst piston engines produce the highest concentration of CO, exhausts from turbine engines also contain CO," it says.

Exposure to CO can lead to damage to the brain, heart and nervous system and, the report adds, "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 AAIB said it was working with the aircraft and engine manufacturers and the National Transportation Safety Board in the US to identify how the gas might have entered the cabin of an aircraft such as the Piper Malibu.

"Work is also continuing to investigate pertinent operational, technical, organizational and human factors which might have contributed to the accident," the report concludes.

Questions raised

Sala's body was recovered from the plane's wreckage on February 6 after a private search for the missing footballer and the pilot was commissioned by his family -- financed by a crowdfunding campaign. The investigation into the crash is ongoing.

Daniel Machover, of Hickman and Rose Solicitors, who represent the Sala family, said in a statement: "That dangerously high levels of carbon monoxide have been found in Emiliano's body raises many 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."