The report, from Ohio's prison system, raised the possibility that Castro died as a result of autoerotic asphyxiation, in which people achieve sexual satisfaction while choking themselves into unconsciousness.
Castro's pants and underwear were around his ankles when he was found, the report said.
He didn't 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've taken his own life, according to the report.
In fact, the day Castro died, the warden had 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 didn't do at least eight required checks on Castro the afternoon and evening before his death and falsified the logs, the report said. Two checks were done just before Castro died.
The report also said staff failed to make sure Castro watched a suicide prevention video when he 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 Thursday's report came out, 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.
"Castro committed heinous and intolerable crimes that required lifetime punishment, but correctional facilities are still responsible for protecting the health and safety of all of the inmates," defense attorney Craig Weintraub said in an email.
The prison guards union accused the state of scapegoating employees instead of dealing with overcrowding and violence behind bars.
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."
Castro's background made him a candidate for attempting autoerotic asphyxiation, said a forensic psychologist, Bob Stinson. But Castro also was a suicide risk as he began a life sentence for a high-profile crime in an environment hostile to sex offenders, Stinson said.
Andrew Welsh-Huggins can be reached on Twitter at https://twitter.com/awhcolumbus