Footballer Emiliano Sala was killed when the unlicensed pilot of his plane lost control and flew too fast as he tried to avoid bad weather, British aviation investigators said on Friday.
The Air Accidents Investigation Branch (AAIB) said in its final report into the 21 January 21 2019 crash that the single-engine Piper Malibu aircraft suffered an in-flight break-up and plunged in to the water, north of Guernsey, upside down.
The AAIB also confirmed that pilot David Ibbotson, 59, was unlicensed to fly that particular aircraft and at night, when the tragedy occurred.
He was also operating an unlicensed commercial flight and being paid an unspecified amount for the flight, which was illegal.
Investigators said it was likely that he was affected by carbon monoxide.
An interim report also found that Sala had suffered carbon monoxide poisoning before the plane crashed and was likely unconscious when it hit the water.
Geraint Herbert, Senior Inspector of Air Accidents at AAIB, said the plane was flying erratically in its last moments as the pilot tried to avoid bad weather.
"During this accident the pilot lost control of the aircraft during a turn, which we believe he was making to avoid poor weather," Herbert told AFP.
"After that the aircraft broke up in flight because it was manoeuvring at a speed well in excess of its maximum manoeuvring speed."
The plane was travelling at an estimated 270 miles per hour (435 kilometres per hour) when it hit the water, said the AAIB.
Investigators said the impact was "not survivable".
Although there was no single cause for the accident, said investigators, the tragedy has thrown a spotlight on lax regulation.
The 115-page report highlighted concerns over the growing use of unlicensed commercial flights, such as the one used to transport Sala, by the "sporting world".
The AAIB also recommended that carbon monoxide monitors be fitted as standard on all planes.
The plane, built in 1984, did not have such a monitor and it is believed that the poisoning was caused by a faulty cabin heater.
Herbert said two main factors contributed to the crash.
"One was that the pilot was not experienced and qualified to fly the flight, but the other one of course was that he was probably suffering from carbon monoxide poisoning," he said.
Sala had signed for Cardiff in a deal completed just a few days before the crash.
The Argentinian was returning to Cardiff to take part in his first training session after saying goodbye to his teammates in northwest France.
In a statement, Cardiff City said it "welcomed" the report which "raises a number of new questions which we hope will be addressed during the inquest recommencing next week".
That inquest is due to start next Monday in Bournemouth, southern England.
The European Business Aviation Association said the tragedy and report had highlighted "the risks of illegal charter flights".
Reports in the British press have said former football agent Willie McKay said he paid for the fatal flight, but did not choose the pilot or the plane.
Sala's body was found several days after the crash, attached to wreckage from the plane, while Ibbotson's body was never found.