Mike Tyson's four-year-old daughter was hospitalized on life support on Monday after getting her neck caught in a cable on an exercise treadmill at home, police said.
The girl, identified by local media as Exodus Tyson, was found by her seven-year-old brother on Monday morning.
The girl's mother, summoned from another room, then freed her, called emergency services and tried to resuscitate her.
"Initial information developed on Tuesday from officers, initial interviews and a preliminary examination of the scene indicate this was a tragic accident," said police spokesman Andy Hill, who said the girl was in "extremely critical condition" on life support.
"Somehow she was playing on this treadmill, and there's a cord that hangs under the console - it's kind of a loop," Hill said. "Either she slipped or put her head in the loop, but it acted like a noose, and she was obviously unable to get herself off of it."
Former world heavyweight champion Tyson, who doesn't live at the house, had been in Las Vegas but travelled to Phoenix upon learning of the accident, Hill said.
Local television station KNXV-TV reported he arrived at St. Joseph's Hospital in a taxi and made no comment as he hurried to the child's bedside.