A report has revealed that Emiliano Sala had been exposed to high levels of carbon monoxide.
The pair crashed on January 21 during a flight from the French city of Nantes to Cardiff, following Sala’s transfer to the club.
Toxicology tests on Sala’s body showed CO levels in his blood so great that it could have caused a seizure, unconsciousness or a heart attack.
Mr Ibbotson has still not been found.
But the Air Accidents Investigations Branch (AAIB) said it is likely he too would have been exposed to carbon monoxide.
Both men are believed to have been exposed to the gas before the plane crashed.
"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, said.
"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."
Sala's blood had a COHb (carboxyhemoglobin) level of 58% - which could cause symptoms including seizure, unconsciousness and heart attack, the report said.
"A COHb level of more than 50% in an otherwise healthy person is generally considered to be potentially fatal," it read.
Lawyer for the Sala family, Daniel Machover of Hickman & Rose solicitors, believes the report raises further questions, such as how the carbon monoxide entered the cabin.
"The family believe that a detailed technical examination of the plane is necessary," he 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."
Sala had just completed a £15 million move to Cardiff when the plane carrying him lost contact with radar near Guernsey.News Now - Sport News