UPDATE, June 4, 2016, 12:35 a.m.: Muhammad Ali has died. He was 74.
Muhammad Ali remained hospitalized Friday in the Phoenix area, battling respiratory problems serious enough to draw family members to his bedside.
The 74-year-old boxing great's respiratory issues have been complicated by the Parkinson's that he was diagnosed with in the 1980s, two people told The Associated Press a day earlier.
The two spoke separately in describing Ali's condition as being very concerning to family members. They declined to be identified because they were not speaking on behalf of the family.
Several of Ali's daughters reportedly flew to Phoenix late Thursday and early Friday to be with their father. Several of his children did not respond to messages from the AP Friday.
A spokesman for the former heavyweight champion said in an email Friday that there was no update on his condition.
Muhammad Ali knocks challenger Ken Norton back with a right hand punch, Sept. 28, 1976. (Photo: The Associated Press)
Ali's longtime Parkinson's doctor declined comment when reached by the AP Thursday night.
"I can't really say much more than what's in the papers," said Dr. Abraham Lieberman of the Barrow Neurological Institute in Phoenix.
Ali has been hospitalized several times in recent years, most recently in early 2015 when he was treated for a severe urinary tract infection initially diagnosed as pneumonia.
Ali has looked increasingly frail in public appearances, including April 9 when he wore sunglasses and was hunched over at the annual Celebrity Fight Night dinner in Phoenix, which raises funds for treatment of Parkinson's.
His last formal public appearance before that was in October when he appeared at the Sports Illustrated Tribute to Muhammad Ali at The Muhammad Ali Center in his hometown of Louisville, Kentucky, along with former opponents George Foreman and Larry Holmes.
Muhammad Ali tours the Muhammad Ali Parkinson Center in Phoenix. (Photo: Ross D. Franklin/AP)
Ali has suffered from Parkinson's for three decades, most famously trembling badly while lighting the Olympic torch in 1996 in Atlanta. Despite the disease he kept up a busy appearance schedule until recently, though he has not spoken in public for years.
Doctors say the Parkinson's likely was caused by the thousands of punches Ali took during a career in which he travelled the world for big fights.
An iconic figure who at one point was perhaps the most recognized person in the world, Ali has lived quietly in the Phoenix area with his fourth wife, Lonnie, whom he married in 1986.
News of his hospitalization brought well wishes from boxers and others on Twitter, including Sugar Ray Leonard, who modelled his career after Ali's.
"Prayers & blessings to my idol, my friend, & without question, the Greatest of All Time ?MuhammadAli ! #GOAT," Leonard wrote.
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