Police Sgt. Andy Hill said the "tragic accident" happened Monday morning at Tyson's home when the girl's 7-year-old brother found her on a treadmill with her neck on a cable attached to the exercise machine.
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The boy told the girl's mother from another room. She took the girl off the cable, called 911 and tried to revive her.
Hill said the girl is now in "extremely critical condition" and on life support.
Hill said Mike Tyson was out of town but returned to Phoenix immediately after learning of the accident.
Calls to Tyson's representatives were either not returned or not answered.