TOKYO — A lawyer for the world’s longest-serving death row inmate - who was acquitted in a Japan ese retrial last week of a 1966 quadruple murder - said Tuesday that the defense team is considering filing a damage suit against the government over the fabrication of evidence that ruined the man’s life and his mental health by keeping him in prison for 48 years.

Iwao Hakamada, an 88-year-old former boxer, was found not guilty last Thursday by the Shizuoka District Court which concluded that police and prosecutors collaborated in fabricating and planting evidence against him. The court said he was forced into confession by violent, hours-long closed interrogations.

The acquittal made him the fifth death row inmate to be found not guilty in a retrial in postwar Japan, where prosecutors have a more than 99% conviction rate and retrials are extremely rare.

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Hakamada was convicted of murder in the 1966 killing of an executive and three of his family members, and setting fire to their home in central Japan. He was sentenced to death in 1968 but was not executed, due to the lengthy appeal and retrial process in Japan’s notoriously slow-paced criminal justice system.

He spent more than 45 years on death row - making him the world’s longest-serving death row inmate, according to Amnesty International.

Hakamada is entitled to receive compensation of up to about 200 million yen ($1.4 million) when prosecutors accept the ruling, making the acquittal final. His lawyer Hideyo Ogawa told reporters that the defense team is also considering filing a damage suit against the government because investigators and the police collaborated in fabricating evidence, despite knowing fully well that it could send the man to the gallows and that would be “totally unforgivable.”

Ogawa also demanded that a recording of the investigation process should be made mandatory in the future.

Hakamada’s 91-year-old sister Hideko Hakamada said she has been trying to explain the victory to her brother but he seems still not convinced that he is now a free man.

The sister, who devoted nearly half of her life to winning her brother’s innocence, said when she told him about his acquittal, as soon as she returned home, he was silent. The next morning, she showed him newspaper stories about him.

“I told him, ‘You see, it’s true, what you kept telling us really came true,’ but he seemed still skeptical,” she said, citing his mental issue and deep suspicion due to his years-long solitary confinement for the crime he was wrongfully accused. “I will keep reminding him of his acquittal every day” until he can finally believe it.

On Sunday, Hakamada, accompanied by his sister, joined his cheering supporters in a meeting in Shizuoka near his hometown of Hamamatsu, in his rare public appearance, and even made a short comment.

“Finally, I have won full and complete victory. Thank you,” Hakamada said. His sister said it was a big surprise as she thought if he could say thank you that was good enough. She says she thinks he is still not fully convinced.

It took 27 years for the top court to deny his first appeal for retrial. His second appeal for a retrial was filed in 2008 by his sister, and that request was granted in 2014, when a court ruled there was evidence suggesting he was wrongfully accused.

The court did not clear his conviction but released him from his solitary death-row cell, allowing him to await retrial at home because his poor health and age made him a low risk for escape. The case has since bounced along several courts until Thursday.

Since his release, he seemed to be in his own imaginary world, and “I never expected him to say such a thing,” Hideko Hakamada said, referring to his remark Sunday. “I imagine he must have rehearsed the phrase while in prison for 48 years so he can say it when he wins acquittal one day.”

The case is not fully closed for them yet because prosecutors can still technically appeal the decision, while his lawyers and human rights activists are condemning such a move and have started a petition drive. It also sparked calls from law associations and rights groups to demand a legal revision to lower hurdles for retrials.

Japan and the United States are the only two countries in the Group of Seven advanced nations that retain capital punishment. In Japan, executions are carried out in secrecy and prisoners are not informed of their fate until the morning they are hanged.