An autopsy at the time did not record a cause of death and Rainey's body was cremated shortly after.
However, the investigation found no evidence that officers at the Dade Correctional Institution regularly used the hot shower to punish or torture inmates, as some of them claimed after Rainey's death.
Assistant State Attorneys Kathleen Hoague and Johnette Hardiman said in the 72-page memo that one inmate's assertions that Rainey was screaming for help and had been scalded to death were unfounded.
"The evidence fails to show that any correctional officer acted in reckless disregard of Rainey's life," they wrote.
Rainey was taken to the shower on June 23, 2012, after he had smeared faeces on himself, the walls of his cell and his bed sheets.
The shower, which was operated from an adjoining room by a corrections officer to prevent inmates from turning it off, was activated but Rainey refused to stand under the water, according to the memo.
Officer Roland Clarke told Rainey he couldn't go back to his cell until he washed off.
Finally, Rainey said he would comply and asked for soap, which he was given, the memo says.
After starting to wash, Rainey said, "No, I don't want to do this," and leaned on a wall away from the water, Clarke told investigators.
Officers continued to check on him, and finally after about two hours the decision was made to take Rainey out of the shower, but he was found lying face up in about 8cm of water with no pulse and not breathing.
One inmate, Harold Hempstead, said he heard Rainey yelling and kicking at the shower door, saying, "I'm sorry. I won't do it any more" and "I can't take it no more."
The prosecutors found Hempstead's claims, which he repeated to several news outlets, were not supported by other evidence, including video footage from inside the prison.
"Hempstead's testimony is inherently unreliable and therefore not credible," Hoague and Hardiman wrote.
Several witnesses said Rainey's skin appeared to be peeled back or reddish in some spots - one inmate claimed he looked like a 'boiled lobster' - but an autopsy found this 'slippage' was most likely caused by friction or pressure on his moist and warm skin.
This could have happened during efforts to revive him, such as chest compressions, or when officers carried him out of the shower initially, the memo said.
The medical examiner, Emma Lew, attributed Rainey's death to a combination of his schizophrenia, heart disease and confinement in the small shower space.
She said schizophrenic people can have nervous system reactions that trigger a heart attack if they have an underlying condition.
"It is not substantiated that the temperatures inside the shower room were excessively high," Lew wrote.
The prosecutors determined that corrections officers did not commit murder or manslaughter in Rainey's death and that taking him to the shower was appropriate under the circumstances.
"Placing an inmate who has defecated upon himself in a shower to decontaminate himself is not conduct that is criminally reckless," they wrote. "There was no evidence of any intent to harm Rainey."
