Ariel Castro, who kept women captive at his home in Cleveland, Ohio, has died after being found hanging in his cell.
Prison officials said he died in hospital late on Tuesday, after prison medical staff failed to revive him.
The former school bus driver abducted Michelle Knight, 32, Amanda Berry, 27, and Gina DeJesus, 23, from the Cleveland streets between 2002-04.
A spokeswoman for the Ohio Department of Rehabilitation and Correction, JoEllen Smith, said: “He was housed in protective custody which means he was in a cell by himself and rounds are required every 30 minutes at staggered intervals.
“Upon finding inmate Castro, prison medical staff began performing life saving measures. Shortly after he was transported to [the prison medical facility] where he was pronounced dead at 10:52 pm.”
“A thorough review of this incident is under way,” she added.
Castro was placed in protective custody because of the notoriety of his case, but was not on suicide watch.
Gina DeJesus was 14 years old when she disappeared. Amanda Berry was 16, and Michelle Knight, 21. Castro also fathered a child with Amanda Berry, who was rescued after she escaped from the house and contacted police.
Castro was arrested the same day. He was charged with multiple counts of kidnapping, rape and aggravated murder.
At his sentencing, Castro told the court that he had been “driven by sex”, adding: “I’m not a violent predator… I’m not a monster, I’m a normal person.
“I’m just sick. I have an addiction, just like an alcoholic has an addiction.”
He said he never planned to abduct the women, but acted on the spur of the moment when he kidnapped his first victim. He said that he was “truly sorry” for what he had done.
But passing sentence, Judge Michael Russo told Castro there was no place in the world for people who enslave others.
Castro pleaded guilty to 937 charges related to the ordeal of his victims, including numerous rapes.
A deal with prosecutors spared him a possible death penalty for murder.
In a victim impact statement made to the sentencing court, Ms Knight told how their captor went to church every Sunday, before coming home to “torture” the women.
“I spent 11 years in hell. Now your hell is just beginning,” she said. “You will face hell for eternity.
“From this moment on, I will not let you define me, or affect who I am. I will live on, you will die a little every day.”
She was the only victim to speak in person at the hearing.
Castro abducted the three between 2002-04.
He lured one of them into his home with the promise of a puppy, and enticed another by inviting her to meet his daughter.
The judge banned Castro from ever seeing the daughter, now six years old.
In an interview last month after his conviction, Castro’s lawyers said that he fitted the profile of someone with a sociopathic disorder. They expressed hope that researchers would study him for clues that could be used to stop other predators.
The two other women were so frightened that at first they were reluctant to leave the house despite the presence of a large number of police officers.
“You saved us! You saved us!” Michelle Knight, told an officer as she ran into his arms.