Exodus was on life support after accidentally hanging herself on a cord dangling from a treadmill in her central Phoenix home. She died at 11:45 a.m., ABC News affiliate KNXV-TV reported, citing police Sgt. Andy Hill.
"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."
Brief footage from KNXV-TV showed the ex-champ arriving at the hospital Monday in a white button-up shirt and black pants, and looking around with a frown before going inside.
Hill said Tyson, 42, had been in Las Vegas but flew to Phoenix immediately after learning of the accident.
"The Tyson family would like to extend our deepest and most heartfelt thanks for all your prayers and support, and we ask that we be allowed our privacy at this difficult time," the boxer said in a statement. "There are no words to describe the tragic loss of our beloved Exodus."
Exodus' 7-year-old brother found her Monday and told their mother, who was in another room. She took Exodus off the cord, called 911 and tried to revive her.
The incident occurred while the child's mother was cleaning the house and the toddler was playing in a playroom, according to a statement from Phoenix police.
When the 34-year-old woman sent her son to see what Exodus was doing, the boy found her "in distress," the police statement said.
He called his mother, who found her on a treadmill with a cable that was attached to the exercise machine wrapped around her neck, police said.
Police said the child's mother ran into the room, freed her from the cord, called 911 and then started CPR.
When police arrived, the girl was unresponsive, and they immediately took over from the woman performing CPR. Fire department workers arrived moments later, and replaced the officers performing CPR, the police said.
The child was eventually taken to St. Joseph's Hospital and Medical Center, where she later died.
Police said that based on the initial investigation, including information from the responding officers, interviews with the mother and son, and a preliminary examination of the scene, it appears that this was "a tragic accident."
Officials from the Phoenix Fire Department told KNXV-TV that the woman who called 911, just after 10:30 a.m. Monday, initially reported an electrocution from a treadmill cord, but then changed her story to say that there had been a hanging.
"He's a really nice guy, definitely a guy I don't want anything bad happening to him, so I hope it is an accident, and I'm pretty sure it probably is an accident, and I hope everything works out," neighbor Shafiq Khalik told the ABC affiliate. "It's very tragic."
"It's terrible," Abby St. Jermain, who grew up in the neighborhood and is house-sitting for her parents down the street, told KNXV-TV. "The kids are so sweet. We always see them riding their bikes, playing. They've just been wonderful people."
Dinka Radic, who lives across the street, described Exodus as smart and sweet.
"The little girl, she says 'You got chocolate in your house?"' Radic said. "I say 'Yes,' and she says, 'OK, give me some."'
When she gives the girl the chocolate, Radic said, "she just kisses me on my knees. Kiss, kiss, kiss. Very nice."
"She was a lively girl," said Ben Brodhurst, who also lives nearby. "She was always making noise. You see her riding a bicycle from house to house, and she was a really eccentric little girl."
The Associated Press contributed to this report.