Muhammad Ali died from septic shock caused by "unspecified natural causes", according to his family.
The sporting legend had battled Parkinson's disease for 32 years before his death on Friday in Phoenix, Arizona.
The 74-year-old had been admitted to hospital on Monday after falling ill with a respiratory condition, family spokesman Bob Gunnell said.
Mr Gunnell said Ali died at 9.10pm local time, surrounded by his family for the last hour of his life.
“Muhammad has battled back many times and we expected it to happen this time,” he said.
“His official cause of death was septic shock due to unspecified natural causes. He did not suffer.”
Writing on Instagram, she said: "No one had even seen anything like it."
She said the family were hugging and kissing him, holding his hands and chanting an Islamic prayer while his heart kept beating as his other organs failed, and called it a "true testament to the strength of his spirit and will".
A funeral will be held in his home town of Louisville, Kentucky on June 10, where flags have been flying at half mast since his death was announced, the family said.
Former US president Bill Clinton and comedian Billy Crystal will deliver eulogies at the service, which will be open to the public. A procession will also be held in Louisville on the same day.
A private family service to celebrate his life will be held on Thursday.
One of his daughters, Hana Ali, said that his heart would not stop beating for 30 minutes after all his other organs failed.
Muhammad Ali - In pictures
Additional reporting by the Press Association.
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