The probe into the Sept. 3 death of Castro includes evidence that he may have died from autoerotic asphyxiation.
An Ohio prisons report on Castro's death released Thursday suggests that possibility, noting his pants and underwear were around his ankles when he was found.
The report also says two guards falsified logs documenting the number of times they checked on Castro before he died.
Castro, 53, was found hanging from a bedsheet just weeks into a life sentence. He pleaded guilty in August to kidnapping three women off the streets, imprisoning them in his home for a decade and repeatedly raping and beating them.
Autoerotic asphyxiation is a sexual act in which people achieve sexual satisfaction while choking themselves into unconsciousness.
Castro did not leave a suicide note, a full psychological evaluation had found no sign he was seriously mentally ill or contemplating suicide, and investigators could find no reason he would have taken his own life, according to the report.
In fact, the day Castro died, the warden recommended he serve his time apart from the other inmates, an option Castro expressed interest in, the investigation found.
The findings were forwarded to the Ohio Highway Patrol "for consideration of the possibility of autoerotic asphyxiation," the report said.
The Highway Patrol said it would have no comment pending the release of its own investigation.
Franklin County Coroner Jan Gorniak, who classified Castro's death as a suicide last week, said Thursday that her office was never told his pants were down. But she said she stands by her finding of suicide.
In Castro's cell, officials found a Bible open to John, Chapters 2 and 3 and pictures of Castro's family arranged "in a poster-board fashion," according to the report.
Surveillance video indicates guards did not do at least eight required checks on Castro the afternoon and evening before his death and falsified the logs, according to the report. Two checks were done properly just before Castro died.
The report also says staff failed to make sure Castro watched a suicide-prevention video when he first arrived in August.
Similar allegations of falsified logs have been made against two other guards in the Aug. 4 suicide of a death row inmate just days before he was to be executed.
After release of Thursday's report, the prison system announced that supervisors will conduct random checks at all prisons to make sure guards are doing their rounds.
Castro's lawyer brushed off the suggestion of autoerotic asphyxiation and said the prison bears responsibility for his death.
The prison guards union accused the state of scapegoating employees instead of dealing with overcrowding and violence behind bars.
The prison report also found that an ambulance was significantly late in arriving but that the delay probably didn't affect the outcome.
In court, Castro blamed his problems on an addiction to pornography. He described himself as a sex addict and said: "I'm not a monster. I'm sick."
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