A civil rights group has stepped up its call for a federal inquiry into Florida’s troubled prison service after a medical examiner reportedly ruled the death of an inmate who was locked under a scalding shower by prison guards for almost two hours an accident.

Darren Rainey, who was schizophrenic, died in June 2012 after he was confined to a tiny cubicle at the psychiatric unit of the Dade correctional institution near Homestead, with corrections officers controlling the water temperature from outside. Several inmates reported hearing Rainey, 50, screaming to be let out while another later claimed he was made to scrape chunks of the dead prisoner’s burned-off flesh from the cubicle floor.

The long-awaited postmortem report from the Miami-Dade medical examiner’s office, however, states that no burns were found anywhere on Rainey’s body, and that his death was caused by complications of his mental illness and heart disease, according to the Miami Herald, which said it spoke to law enforcement sources close to the case.

Howard Simon, executive director of the Florida chapter of the American Civil Liberties Union (ACLU), said it “defies belief” that Rainey, who was serving a two-year sentence for cocaine possession, and who had been locked in the shower after he defecated in his cell, died by accident.

“To accept the medical examiner’s conclusion you have to believe that he accidentally locked himself in a shower, then turned up the water temperature to 180 degrees, accidentally boiled himself to death and all the while he was screaming for help,” he told the Guardian.

“That doesn’t sound to me much like an accident.”

Rainey’s death led to a prisons department investigation into alleged abuse at the Dade facility. Jerry Cummins, the warden, was fired in 2014 and two officers who were on duty on 23 June 2012 resigned.

Following receipt of the autopsy report, the Miami-Dade state attorney’s office will now decide if any criminal charges are to be brought against any of the prison officers. But Simon said he had little confidence in the process and renewed a previous demand made by the ACLU, Amnesty International, the Florida Council of Churches and other rights groups for a full investigation by the US Department of Justice. Department officials confirmed “an ongoing criminal investigation” into Rainey’s death last March, but Simon said there have been no formal developments since.

“There needs to be an independent assessment,” he said. “The final autopsy by the medical examiner needs to be released and subjected to an independent assessment, and ultimately you need the federal government in here to analyse what went wrong.”

“The key word is ‘independent’. The system here is that there’s too much of a history of protecting their own. It may be that there was a homicide here.”

A spokesperson in the office of Dr Bruce Hyma, the Miami-Dade medical examiner, said his office was not releasing any details of the postmortem, including why it took more than three and a half years to reach a conclusion.

But according to the Miami Herald, the autopsy concluded that while Rainey’s “confinement” contributed to his death, no thermal injuries or burns were found on his body. The alleged melting off of Rainey’s skin was explained by a condition known as “slippage” caused by prolonged exposure to water and the warm, moist environment in the shower cubicle, the newspaper quoted its sources as saying.

It also claimed that a nurse recorded Rainey’s body temperature at 102F, almost four degrees above normal, as staff administered CPR, and that his body was still at 94F almost 12 hours after his death.

In addition to inquiries by the inspector general of Florida’s department of corrections and the Miami-Dade police department, the Miami Herald interviewed numerous inmates during a wider investigation into questionable deaths, beatings and other abuses in the state’s prisons over many years. One inmate, Harold Hempstead, a convicted burglar who was acting as an inmate orderly when Rainey died, claimed officers at DCI routinely taunted and terrorised mentally ill prisoners and had doctored the shower to be used as a punishment.

Numerous other cases were looked into, including the September 2010 death of Randall Jordan-Aparo, a 27-year-old prisoner who suffocated when he was gassed by guards in an isolation cell at Franklin correctional institution in the Florida panhandle. An inquiry by the Florida department of law enforcement initially ruled that officers were blameless.

In a darker side to that inquiry, three senior corrections department investigators sent to Franklin to look into Jordan-Aparo’s death and other serious incidents claimed their findings of systemic, widespread corruption and abuses were ignored, and that they later found themselves under investigation. After giving “whistleblower” evidence to state legislators, the three claimed in a lawsuit against the department that they were sidelined and transferred to non-investigational desk duties, “the clearest case of retaliation I’ve seen in my 37 years of practicing law”, according to their attorney, Steven Andrews.

Julie Jones, Florida’s secretary of corrections, meanwhile, dismissed the inspectors as a “group of disgruntled employees that do not have the department’s best interests at heart”.

McKinley Lewis, communications director for the Florida department of corrections, said on Monday that he did not believe Rainey’s autopsy report had been received but that when it was it would be reviewed thoroughly.

“Of course, the medical examiner’s report is not the be all and end all,” he said. “There’s the Miami-Dade police report to come and comments from the state attorney’s office. We’ll take a comprehensive look and make sure that our policies and procedures are in compliance with any recommendations made. If there are any deficiencies or areas of concern we can address them.”

For Simon, the ACLU director, Rainey’s death highlights another problem in Florida. “The question has to be raised as to why Darren Rainey was in prison in the first place,” he said.

“That’s maybe as important as the issue of how he was treated and how he came to die. Why would the state of Florida send somebody like him, with a history of mental illness, to prison for the possession of drugs? That’s not what prisons are supposed to be for.”