Footballer Emiliano Sala was exposed to harmful levels of carbon monoxide before he was killed in a plane crash and it is likely his pilot was also affected, according to accident investigators.
Tests on the striker's body found enough evidence of the harmful gas to cause a heart attack, seizure or unconsciousness, an interim report by the Air Accidents Investigation Branch (AAIB) stated.
The cockpit of the Piper Malibu aircraft involved in the crash was not separated from the cabin and it is "likely" that pilot David Ibbotson was also "affected to some extent" by exposure to carbon monoxide, the document added.
The AAIB said the gas can "reduce or inhibit a pilot's ability to fly an aircraft depending on the level of that exposure".
Argentinian footballer Sala signed for Cardiff City from French club Nantes for £15 million on 18 January.
Mr Ibbotson, 59, of Crowle, Lincolnshire flew Sala from Cardiff to Nantes the following day.
The return flight - which crashed in the Channel - was on 21 January.
Sala's body was recovered on February 6 but Mr Ibbotson has not been located.
The aircraft remains underwater off the coast of Guernsey after an attempt to recover it was hampered by bad weather.
The AAIB said it was working with the aircraft and engine manufacturers and the National Transportation Safety Board in the US "to identify possible pathways through which CO might enter the cabin of this type of aircraft.
"Work is also continuing to investigate pertinent operational, technical, organisational and human factors which might have contributed to the accident."
Piston engine aircraft such as the Piper Malibu involved in the crash produce high levels of carbon monoxide, the interim report said.
The gas is normally conveyed away from the aircraft through the exhaust system but poor sealing or leaks into the heating and ventilation system can enable it to enter the cabin.
Several devices are available to alert pilots over the presence of carbon monoxide.
The AAIB said they are not mandatory but can "alert pilots or passengers to a potentially deadly threat".
Investigators have previously said the validity of Mr Ibbotson's licence will form a key part of their inquiry.
The type of licence he held meant he could only fly passengers in the European Union on a cost-sharing basis, rather than for commercial flights.
A full accident report is expected to be published next year.
- WATCH: The AAIB has published a second Special Bulletin on the loss of Piper Malibu aircraft...
The wife of David Ibbotson - the pilot killed alongside footballer Emiliano Sala - has told ITV News she trusts investigators to find out what happened to the pair.
The footballer's family is calling for the plane's wreckage off Alderney to be salvaged by air accident investigators after new evidence emerged earlier this week.
But in an exclusive interview with Good Morning Britain, Nancy Ibbottson said she is now unsure what that will achieve.