The pilot of the plane that crashed killing footballer Emiliano Sala was not licensed to fly the aircraft, a report has found, reports BBC Sport.
The 28 year old and pilot David Ibbotson died in the crash in the English Channel, two days after the Argentine signed for Cardiff City in January 2019.
The Air Accidents Investigation Branch published its findings on Friday.
It said Sala would have been "deeply unconscious" from carbon monoxide poisoning at the time.
Chief Inspector of Air Accidents Crispin Orr said it had been a "long and complex" investigation, and the Civil Aviation Authority (CAA) was probing "whether there were any breaches of the Air Navigation Order during the flight".
Cardiff City FC said the report was an "important step" which raised a "number of new questions which we hope will be addressed".
"We are encouraged to read that the CAA is determined to tackle illegal activities by pursuing those involved," the club said in a statement.
"It is a practice which must be stopped and we hope the industry will be supported in order to prevent this tragedy ever happening again."
Mr Ibbotson lost control of the plane while descending to avoid cloud and he was probably also affected by carbon monoxide, the Air Accidents Investigation Branch (AAIB) concluded.
The plane began to break up in mid-air as the pilot tried to regain control, investigators found.