New Zealand team doctor John Mayhew says legend Jonah Lomu died of a heart attack.
Lomu passed away on Tuesday aged just 40 after suffering from a long history of kidney problems.
"The final mechanism was something caused the heart to go into cardiac arrest, most probably a cardiac or pulmonary event," Mayhew told the New Zealand Morning Herald.
"Cardiac arrest is the final pathway of the heart shutting down. That's all there is really."
Rugby changed forever after Jonah Lomu's performance in the New Zealand semi-final with England in 1995.
Former England head coach Sir Clive Woodward says it was a privilege to coach against the iconic Jonah Lomu.
Former England fly-half Rob Andrew says Jonah Lomu was "a gentle giant, on and off the pitch".