Hamamatsu (CNN) Sitting in an armchair next to his sister, in a cozy living room in the Japanese city of Hamamatsu, Iwao Hakamada looks like your average 83-year-old grandpa.

But in 2014, he became the world's longest-serving death row inmate, after spending nearly five decades in a tiny, solitary cell waiting for the hangman's call.

In 1966, the former professional boxer-turned-factory worker was accused of robbery, arson and the murder of his boss, his boss' wife and their two children. The family was found stabbed to death in their incinerated home in Shizuoka, central Japan.

Iwao initially admitted to all charges before changing his plea at trial. He was sentenced to death in a 2-1 decision by judges, despite repeatedly alleging that police had fabricated evidence and forced him to confess by beating and threatening him. The one dissenting judge stepped down from the bar six months later, demoralized by his inability to stop the sentencing.

A pair of blood-spattered, black trousers and his confession were the evidence against Iwao. The alleged motive ranged from a murder by request to theft.

victims' blood type. But in 2004, a DNA test revealed that blood on the clothing matched neither Iwao nor thevictims'blood type.

In 2014, the Shizuoka District Court ordered a retrial and freed Iwao as he awaited his day in court, on the grounds of his age and fragile mental state. But four years later, the Tokyo High Court scrapped the request for a retrial, for reasons it would not confirm to CNN.

That means Iwao could go back to prison and face the death penalty -- again.

His legal team has launched an appeal to get a retrial and is waiting to hear from the Supreme Court.

But in Japan, where the criminal justice system has a 99.9% conviction rate, clearing his name will not be easy.

Unexpected arrest

The youngest of six siblings, Iwao grew up in the seaside city of Hamamatsu, around two hours from Tokyo by train.

The family was poor but enjoyed a happy and stable environment, says Hideko Hakamada, Iwao's 86-year-old sister, who has campaigned to clear his name. Iwao now suffers from mental illness brought on by decades of imprisonment.

As children, the siblings went fishing by the seaside during summer and roasted garlic cloves amongst the fallen leaves in their yard in autumn. Only three years apart in age, Hideko and her little brother -- who she describes as calm and quiet -- were close. "He was like my shadow. He would follow me around everywhere," Hideko remembers.

Photos of Iwao Hakamada spread along the tatami mat at his home in Hamamatsu.

As the pair grew up, Hideko got married and Iwao started working in a bodybuilding gym. A colleague encouraged him to take up boxing.

and went on to fight 29 bouts. After his conviction, he was dubbed the "Japanese Rubin Carter" by some In 1959, at the age of 23, he started his professional career as a featherweight boxerand went on to fight 29 bouts. After his conviction, he was dubbed the "Japanese Rubin Carter" by some Western media . Carter was an American-Canadian boxer who was wrongfully convicted of murder but freed after serving almost 20 years in prison.

In 1961, Iwao retired after he fell ill and got a job as a factory worker at a soybean processing plant in Shizuoka. At the time, he was divorced and had a son.

On June 30, 1966, the news broke that Iwao's boss and family had been murdered in the early hours of the morning.

From the start, the odds were stacked against Iwao. As a former boxer and divorcee who had also worked in a bar, the police considered him a low-life and the most likely suspect, says Tsunogae.

On July 4, 1966, the police brought Iwao in for questioning.

Iwao Hakamada was briefly a professional boxer who fought 29 bouts.

At the time, many media outlets falsely reported that a blood-stained shirt found at the scene of the crime belonged to Iwao. The police let him go due to a lack of evidence and continued their investigations for a month.

After coming to a dead end, officers circled back to interviewing Iwao and his family members. Tsunogae alleges the police were desperate for a suspect and keen to pin the crime on Iwao.

They were taken to the police station and interrogated separately from morning till nightfall. Back at their mother's house, the family swapped notes about what they'd each been asked.

But Iwao was kept in solitary confinement at the station with no access to legal counsel. His ordeal was just beginning.

99% conviction rate

Japan puts far fewer people in prison than most developed countries: 39 per 100,000 people, compared with 655 in the US and 124 in Spain, according to the World Prison Brief website.

But in the US, for example, alleged offenders are entitled to remain silent during police questioning, have the right to legal representation both outside of and during questioning, and can make a deal to admit a less serious charge. This allows the prosecutor to drop a case and file an indictment from, for example, first degree murder to second degree murder.

By contrast, in Japan the accused can remain silent during police questioning, but suspects can be interrogated without the presence of a lawyer.

Iwao Hakamada and his sister Hideko (R) at their home in Hamamatsu.

Though plea bargains were introduced in 2016, their use is limited to categories such as corporate malfeasance, according to Colin Jones, a law professor at Doshisha University, although defendants who cooperate can expect a more lenient sentence.

The 99% conviction rate is partly because prosecutors only pursue cases they believe will lead to a guilty verdict, according to Mark Ramseyer, a law professor at Harvard University.

"There are very few prosecutors in Japan. They are massively overworked. Given the workload, they focus first on the slam dunk, guilty-as-sin cases. They don't have time for the 'maybe he did it, maybe he didn't cases,'" says Ramseyer.

And because crime rates are so low in Japan, if someone is arrested or prosecuted, people generally assume they have done something wrong, says Jones.

"They only realize Japan's criminal justice system is not functioning when they are personally caught up in it," he says.

'Hostage justice' system?

Hideko says her brother fell victim to "hostage justice," when police allegedly strip suspects of their right to remain silent and coerce them to confess.

was subject to more than 240 hours of questioning for over 20 days, according to Kiyomi Tsunogae, a member of Iwao's defense team. The police and prosecutors initially Hideko claims there are no recordings that provide evidence that he confessed to mass murder. Hewas subject to more than 240 hours of questioning for over 20 days, according to Kiyomi Tsunogae, a member of Iwao's defense team. Thepolice and prosecutors initially alleged Iwao had been having an affair with his boss' wife and that she had asked him to murder her family and burn their house down so they could start a new life together. But officers discarded that theory after two days, she says.

Towards the end of the trial, they said he had wanted money.

Police in Japan can keep suspects detained for up to 23 days and repeatedly extend this deadline by filing new accusations. CNN asked the Japanese police whether the allegations that they used forced confessions during interrogations was true, but they provided no comment.

Confessions have long been the cornerstone of Japanese criminal justice. They are considered "the king of evidence" and "decisive element of proof" sought by every prosecutor and expected by most judges, David T. Johnson, an expert on the death penalty in Asia, wrote in a paper published in 2015.

In most democracies, a confession obtained after more than 200 hours of interrogation would be ruled as involuntary, unreliable, and inadmissible as evidence, adds Johnson. But not so in Japan.

In Iwao's case, police and prosecutors composed 45 confession statements, which they presented to the judges. Only one was accepted to be admissible as evidence.

In January, Nissan's Carlos Ghosn, who fled criminal prosecution in Japan, pointed to his time in solitary confinement and blasted the Japanese legal system, saying it "violates the most basic principles of humanity."

Japan's Justice Minister Masako Mori -- who will host the UN's Congress on Crime Prevention and Criminal Justice in April -- refuted Ghosn's accusations. She said it was inappropriate to focus and compare particular parts of each country's justice system, without looking at how that system works as a whole.

Some legal experts say Japan's justice system keeps the country safe. Crime rates in 2019 hit a post-war low, according to data released by Japan's National Police Agency.

Iwao Hakamada pictured leaving a Tokyo detention in 2014 after 48 years on death row.

But over the decades, other high-profile cases have highlighted miscarriages of justice.

In 2009, Toshikazu Sugaya was freed in a rare judicial reversal that sent shock waves through the country's justice system. In 1990, he was falsely convicted of the rape and murder of a four-year-old girl following a confession extracted under duress.

The following year, in a separate case which became a scandal , a Japanese prosecutor acknowledged that he had tampered with evidence to secure the conviction of Muraki Atsuko , a health ministry official who was accused of postal fraud . His bosses allegedly knew the prosecutor had tampered with evidence, but tried to cover that fact up.

Since then, there have been reforms to Japan's justice system.

to serve alongside three professional judges in cases involving certain serious crimes. In 2009, Japan introduced a jury-like system where six lay judges are chosen from the electoral rollto servealongside three professional judges in cases involving certain serious crimes.

which include serious charges such as murder. In 2016, mandatory video recordings of interrogations were introduced , but critics say recordings only apply to 3% of the nation's criminal cases --which include serious charges such as murder.

Attempts to make investigators record interrogations have faced resistance from the police and prosecutors, who have tried to limit the scale and scope of those requirements, according to Osamu Niikura, a lawyer and member of the Japan Federation of Bar Associations (JFBA).

Hideko Hakamada, the elder sister of death row inmate Iwao Hakamada speaks to media reporters after the Shizuoka District Court accepted a request for a retrial on March 27, 2014 in Shizuoka, Japan.

Furthermore, acquittals can be seen as detrimental to the careers of both judges and prosecutors.

But in Japan, the price for a miscarriage of justice can be death.

a declaration to abolish the death penalty by 2020, in time for Japan to host the Olympic Games, and to replace it with life imprisonment, in line with most developed nations. It was submitted to the Prime Minister, the Minister of Justice, the Speaker of the House of Representatives, and the President of the House of Councilors but is not legally binding and the death penalty has not been abolished. In 2019, the JFBA approved a declaration to abolish the death penalty by 2020, in time for Japan to host the Olympic Games, and to replace it with life imprisonment, in line with most developed nations. It was submitted to the Prime Minister, the Minister of Justice, the Speaker of the House of Representatives, and the President of the House of Councilors but is not legally binding and the death penalty has not been abolished.

A thorny issue

Hideko used to think the death penalty was a reasonable punishment for those who committed terrible crimes, until she saw the effect its sentencing had on her brother.

For the first 20 years of imprisonment, Iwao believed that one day he would be set free. He applied himself to his studies, found solace in God, and wrote frequent letters home. In each one, he told his mom he was innocent and asked after his son.

Hideko says his optimism gave her hope.

Hideko Hakamada sits in her home in Hamamatsu, Shizuoka, Japan on Monday Dec. 2, 2019. Hakamada worked for years fo are her brother Iwao prisons.

But in 1980, after Japan's Supreme Court finalized Iwao's death penalty, he was transferred to a prison exclusively for death row inmates.

The day after Iwao arrived there, the guards took the man in the cell next to his off to the gallows. "I thought they did that deliberately to break Iwao's spirit," says Hideko. "Iwao told me that that man shouted out his farewells to the other inmates as he was taken away."

Soon after, Iwao's letters home became more sporadic. His once clear handwriting slipped into scrawls. For over a decade starting from the 1990s, Hideko's attempts to see Iwao were rejected.

They were finally able to meet after Hideko appealed to a member of parliament in Tokyo for help. But when the siblings finally reunited, Iwao didn't seem to remember who he was, she says.

A letter that Iwao Hakamada wrote to his mother from prison, claiming his innocence, sits on tatami mat in Hamamatsu.

Unlike in the US where execution dates are set in advance, death row prisoners in Japan are executed in secret, with no advance warning given to the prisoner, their family nor legal representatives, according to Amnesty International.

Prisoners often only learn of their execution hours before it's due to take place. Japan justifies this by saying it is "out of consideration that an advance notice would disturb the inmate's peace of mind and might cause further suffering."

Usually, inmates must be executed within six months of their sentencing hearing. But Tsunogae says this rarely happens, and many end up waiting years.

Capital punishment is usually reserved for those who have committed multiple murders. All executions are carried out by hanging.

In Japan, the public supports the death penalty; 80% of respondents in a 2019 report released by the Cabinet Office said they were in favor. Only 9% opposed it.

'In his own world'

Over the decades, Iwao has garnered a loyal following of supporters.

Artists and sportsmen as well as non-profit groups and volunteers have all joined the call to clear his name, and cast a spotlight on the plight of death row inmates, who like Iwao can end up with mental illness.

In 2018, Iwao's former boxing association launched a petition supporting calls for a retrial. The following year, they commissioned a manga series on Iwao's life, saying boxers have a bad rap in Japan as the sport is not deemed as prestigious as judo and kendo.

That same year, Iwao and his sister traveled to Tokyo to attend a mass given by Pope Francis. Iwao was baptized as a Catholic in 1984 while in prison. His sister had hoped he could meet the Pope, who in 2018 called on all world leaders to abolish the death penalty, but their request went unanswered.

Over the decades, Amnesty International and Iwao's former boxing association have organized a series of small demonstrations in front of Japan's parliament.

Iwao Hakamada buys himself an ice cream as he takes his daily walk around Hamamatsu,

Though Iwao will likely never return to full mental health, Hideko says her brother has gotten healthier and lost the limp he had while on death row. He has a routine: he wakes early and goes for a four-hour stroll along the streets of Hamamatsu with a volunteer. "He's just in his own world now," says Hideko.

Hideko usually gives him 500 yen ($4) so he can buy himself snack on the walk. He sometimes ends up giving it to young children. Iwao has not seen his son since he went to prison, but Hideko suspects that he remembers that he once had a child.

Back at their shared home, Iwao is mostly silent. Once in a while, he contributes a word or two to the conversation before picking a Kleenex, which he neatly folds into squares -- a habit he picked up on death row, says Hideko.

Till this day, it is not clear who killed Iwao's boss, his boss' wife and their two children. Under Japanese law, Iwao is still recognized as a convicted killer under the sentence of death.

For now, Hideko says that the best thing for Iwao would be to remain free. Despite the odds, she is committed to clearing her brother's name and to changing people's perceptions of the death penalty.

"Society has prejudices against people who were on death row," says Hideko. "But someone who committed a crime, or who finds themselves on death row, is still a human."