Ariel Castro, who kept women captive at his home in Cleveland, Ohio, has died after being found hanging in his cell.
Prison officials say he died late on Tuesday, after prison medical staff tried to revive him, the BBC reports.
A spokesperson 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."
She says a thorough review of the incident is under way.
Castro was placed in protective custody because of the notoriety of his case but was not on suicide watch, the BBC reports.
He was sentenced last month to life in prison for abducting, raping and holding captive three women for as long as 11 years, and murder for forcing one of the women to abort her pregnancy.
Michelle Knight, 32, Amanda Berry, 27, and Gina DeJesus, 23, were rescued earlier this year after Ms Berry escaped from the Cleveland home. The house was demolished last month.
Castro pleaded guilty In late July to several hundred counts of kidnapping, rape and aggravated murder to avoid the death penalty. The court heard he kept his victims chained up and repeatedly raped them.
At his sentencing Castro said: "I'm not a monster, I'm a normal person, I am just sick, I have an addiction"