Tokyo prosecutors said Monday they will not appeal a recent high court order granting a retrial to a former professional boxer previously sentenced to death over a 1966 quadruple murder case in central Japan, laying the stage for his exoneration.

With prosecutors deciding not to file a special appeal with the Supreme Court, the retrial of the 87-year-old Iwao Hakamata, who spent nearly half a century behind bars before new evidence led to his release in 2014, is set to be held at the Shizuoka District Court, where he was initially tried.

Iwao Hakamata (L) and his elder sister Hideko are pictured at his home in Hamamatsu, Shizuoka Prefecture, central Japan, on March 20, 2023. (Photo courtesy of a supporters' group for Iwao Hakamata)(Kyodo)

Hakamata's case marks the fifth time in postwar Japan a decision for a retrial has been finalized in a case in which the death penalty had been given. The four previous cases all resulted in acquittals in the 1980s.

The prosecutors likely made the decision due to difficulties in satisfying conditions for a special appeal, which are limited to violations of the Constitution or judicial precedents.

"Though there are some points we find hard to swallow, we could not find grounds for filing a special appeal," said Hiroshi Yamamoto, deputy chief prosecutor of the Tokyo High Public Prosecutors Office in a statement.

In a press conference held in late afternoon, he said the decision was made on Monday after "careful considerations based on law and evidence" but declined to elaborate.

Hakamata's elder sister Hideko, 90, expressed relief at the decision, saying she told her brother to "not worry as the retrial will start."

Hideko, who for 57 years maintained her brother's innocence, visited Hakamata every month despite mental conditions caused by his long incarceration manifesting from around 1980, when his death sentence was finalized.

Hakamata went about his day as usual on Monday, eating fruit for breakfast and gazing at white tulips planted by supporters near his home when he left for his daily walk.

Meanwhile, Katsuhiko Nishijima, who led Hakamata's defense team, said the prosecutors' decision not to appeal was "a given. We want to clear his name in the retrial."

On March 13, the Tokyo High Court, which was ordered by the Supreme Court in 2020 to re-examine its 2018 decision not to reopen the case, reversed course and ordered the retrial, citing the unreliability of the main evidence used.

The high court said there was a strong possibility that the evidence -- five pieces of clothing Hakamata allegedly wore during the incident -- used in finalizing his death sentence had been planted by investigators in a tank of miso soybean paste in which they were found.

Hakamata had always insisted on his innocence, but his sentence was finalized in 1980. He was freed in 2014 after the Shizuoka District Court decided to suspend his death sentence and reopen the case, accepting DNA test results that indicated blood found on the clothing items was not Hakamata's.

But the Tokyo High Court scrapped the Shizuoka court's decision to reopen the case in 2018, questioning the lower court's reliance on the DNA tests.

The Supreme Court upheld the high court decision over the credibility of the DNA tests but concluded the case should be re-examined because questions had not been resolved over the color of alleged bloodstains left on the clothing items.

Indicted for murder, robbery and arson, his death sentence was finalized based on a ruling that blood marks on the clothing items, discovered 14 months after the incident, matched the blood types of the victims and Hakamata. Prosecutors said he had hidden the items immediately after the alleged criminal act.

But the defense team refuted the claim, saying blood stains on the items still retained redness, an indication someone had put them in shortly before their discovery.

The team cited forensic scientists' analysis that the reddish color of blood stains on clothing would turn black when immersed in miso for several months and would not stay red after being soaked for more than one year.

The former boxer was a live-in employee at a miso maker when he was arrested in 1966 for allegedly killing the firm's senior managing director, his wife and two of their children. They were found dead from stab wounds at their house in Shizuoka, which had been burned down.

After his arrest, Hakamata had initially confessed to investigators but pleaded not guilty at his trial.

Lawyers hold up signs in front of the Tokyo High Court on March 13, 2023, after it ordered that the case against Iwao Hakamata, who was sentenced to death over a 1966 quadruple murder case, be retried. (Kyodo)

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Tokyo court orders retrial of 1966 quadruple murder case