Former Nantes star Emiliano Sala and pilot David Ibbotson are "likely" to have been exposed to carbon monoxide before the plane on which they were travelling crashed over the English Channel in January.
A report from the Air Accidents Investigation Branch (AAIB) said toxicology tests on Argentinian Sala, whose body was found in the wreckage, showed clear signs of exposure to the poisonous gas. Search teams have found no sign of Ibbotson, who is presumed dead.
Striker Sala, who was 28, had said goodbye to former Nantes team-mates before travelling to join his new club, Cardiff City, when the Piper Malibu plane crashed north of Guernsey.
He died from head and trunk injuries , an inquest heard in February.
The AAIB said in a statement on Wednesday that its latest findings showed "the danger of exposure to carbon monoxide in both piston and turbine engine aircraft".
"Toxicology tests found that the passenger had a high saturation level of COHb (the combination product of carbon monoxide and haemoglobin)," said the AAIB statement.
"It is considered likely that the pilot would also have been exposed to carbon monoxide. When our investigation has concluded, we will publish a final report."
Geraint Herbert, principal inspector of air accidents with the AAIB, explained the impact of carbon monoxide poisoning.
He said: “Carbon monoxide is an odourless and colourless gas. Symptoms at low-exposure levels can be drowsiness and dizziness, but as the exposure level increases it can lead to unconsciousness and death.
“Inexpensive carbon monoxide detectors are widely available and can be fitted to aircraft to warn pilots of the presence of carbon monoxide.
“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 kind of aircraft.
“We'll be publishing our final report in due course.”
Cardiff says the latest findings show the plane carrying Sala was "not appropriate".
A club spokesperson said: "CCFC is concerned at the AAIB's latest report which once again highlights that the aircraft used for Emiliano Sala was not appropriate. We continue to believe that those who were instrumental in arranging its usage are held to account for this tragedy."
The AAIB, meanwhile, has responded to calls from Sala's family to recover the plane's wreckage.
"The reasons for our decision not to recover the aircraft wreckage have been explained in detail to both families concerned," the organization said in a statement.
"In February our underwater search operation successfully located the wreckage, recovered the passenger's body and captured substantial video evidence from the scene using a remotely operated vehicle. It was not possible at the time to recover the wreckage.
"We have carefully considered the feasibility and merits of returning to attempt to recover the wreckage. In this case, we consider that it will not add significantly to the investigation and we will identify the correct safety issues through other means.
"In making our decision, we took into account the high cost of underwater recovery, the evidence we collected in February and the risk that, after a violent impact with the sea, the wreckage would not yield definitive evidence."