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.