Prison spokeswoman JoEllen Smith is cited as saying that Ariel Castro was found hanging in his cell at 9:30pm (2:30am UK time) last night.
Prison medical staff performed CPR before he was transported to a hospital, where he was pronounced dead, she said.
Castro was sentenced to life in prison plus 1,000 years after pleading guilty to 937 counts including kidnapping and rape.
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The 23-month-old continued breathing after life-support was withdrawn and doctors agreed to give him oxygen and water, his father said.
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