Story highlights Iwao Hakamada served 48 years in prison for murders of his boss and boss's family

Court granted him a retrial, saying some evidence may have been fabricated

Amnesty international says he was beaten and threatened into confessing in 1966

78-year-old ex-boxer's mental state deteriorated because of isolation, Amnesty says

Iwao Hakamada holds the most dubious of records: Convicted of a 1966 quadruple murder, he is the world's longest-serving death row inmate, according to Amnesty International.

That record was capped Thursday when, after almost 48 years, a local court reopened Hakamada's case, Amnesty and Japanese media reported. The Shizuoka District Court suspended his death sentence and released Hakamada after DNA testing indicated key evidence against him may have been fabricated, reported NHK, the Japanese public broadcasting organization.

The court said keeping him detained any longer would be unjust, NHK reported

Prosecutors have four days to appeal the court's ruling, but Amnesty International's East Asia research director said it would be "most callous and unfair" to challenge the court's decision.

"Time is running out for Hakamada to receive the fair trial he was denied more than four decades ago," Roseann Rife said . "If ever there was a case that merits a retrial, this is it. Hakamada was convicted on the basis of a forced confession, and there remain unanswered questions over recent DNA evidence."

NHK video showed several men in blue uniforms, some wearing surgical masks, escorting the 78-year-old former pro boxer out of a Tokyo detention center Thursday.

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Hunched forward and wearing a yellow, short-sleeved button-down, Hakamada entered a silver van. He was accompanied by his sister, Hideko Hakamada, who earlier attended a rally outside the court in which several dozen supporters broke into emphatic applause when a man unfurled a sign saying, "Retrial granted."

An emotional Hideko Hakamada told the crowd, "Thank you very much. I am very pleased. I am very thankful with everyone's support," according to the NHK footage.

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Iwao Hakamada was convicted in 1966 of killing his boss at a soybean processing company, along with the boss's wife and two children, Amnesty said.

"Hakamada 'confessed' after 20 days of interrogation by police. He retracted the confession during the trial and told the court that police had beaten and threatened him," Amnesty said. "According to his lawyers, recent forensic tests show no match between Hakamada's DNA and samples taken from clothing the prosecution alleges was worn by the murderer."

District Court Judge Hiroaki Murayama, who handled the retrial proceeding, said DNA test results indicated blood found on five items of clothing allegedly worn by the culprit was not Hakamada's, according to Kyodo News

Like most death row inmates in Japan, Hakamada was largely held in solitary confinement during his 48 years in prison. His mental health has deteriorated as a result of the decades he spent isolated, Amnesty said.

Hakamada's case marks the sixth time a death row inmate in Japan has earned a retrial, and courts overturned death sentences in four of the five previous cases, NHK reported.