When a man convicted of a 1948 mass murder died of natural causes on death row 25 years ago, most people believed that meant unresolved questions about the “Teigin Incident” would never be answered.

However, a group of lawyers and experts in pharmacology and psychology are still struggling to unearth the truth behind the notorious mass-poisoning case and exonerate Sadamichi Hirasawa, who passed away May 10, 1987, in a prison hospital in Tokyo at age 95.

“We will complete presenting new evidence by the end of next year to reopen the case,” said Nobuyoshi Araki, one of the lawyers involved in the petition for a posthumous retrial of Hirasawa, who was an artist.

The 19th petition was filed with the Tokyo High Court by Hirasawa’s adopted son, Takehiko, on May 10, 1989, the second anniversary of his death. The first was filed in 1955 and rejected the next year.

One of the focuses in the petition is on the poison used to kill the 12 victims at a branch of Teikoku Ginko (Imperial Bank, also known as Teigin) in Toshima Ward, Tokyo, on Jan. 26, 1948, by a man posing as a health official.

Telling the people in the bank that dysentery had broken out in the neighborhood, the man gave them a drug, saying it was a “remedy.” It turned out to be poison. He escaped with cash and checks.

While the courts determined the murderer used potassium cyanide, Hiroyoshi Endo, former dean of the pharmacology department at Teikyo University, said, “I can’t specify what the poison was, but as a scientist I can say it was not potassium cyanide.”

The victims would have suffered immediate effects from potassium cyanide, but a certain amount of time passed before they died, indicating the poison had a delayed effect, he said.

Endo told a public gathering held May 10 in Tokyo to commemorate the anniversary of Hirasawa’s death that such a poison with delayed effects “could not have been used by a painter, who had little chemical knowledge.”

“Thus, I strongly believe it was impossible that Mr. Hirasawa was involved in the Teigin Incident,” he said.

Endo has presented these arguments in written expert opinion submitted to the high court.

Meanwhile, psychologists are working to determine if the testimony of witnesses identifying Hirasawa as the murderer and his confessions during questioning are credible by reviewing investigation and court records.

“We have carried out a series of experiments to examine if people could identify the same person after an interval of several months, and confirmed that their memories are rather precarious,” Yukio Itsukushima, a psychology professor at Nihon University, told the Tokyo gathering.

Itsukushima, who tested people’s ability after a lengthy interval to pick out people in lineups such as the one police used for Hirasawa, also said witnesses tended to be influenced by external factors such as what other witnesses said.

Hirasawa, who was arrested in August 1948, eventually confessed to the crime, but he later retracted the confession and pleaded not guilty in court.

The psychologists said they are examining Hirasawa’s admission of guilt at a time when investigators relied on confessions, often forced, rather than objective evidence.

“It is said that Mr. Hirasawa had a delusive tendency and was a habitual liar, and we think it is necessary to examine if such a morbid tendency contributed to causing the transition in his statements during investigation,” attorney Araki said.

The lawyers plan to check if his history of health problems, including the neurological disorder Korsakoff’s psychosis, affected his personality development.

“There is no evidence anyway to clearly support the allegation that Mr. Hirasawa committed the crime,” Araki added.

The ongoing petition for a retrial is chiefly based on a memo hundreds of pages long written by Bunsuke Kai, a Tokyo police investigator who was involved in the case.

The Kai memo indicates that investigators initially believed someone connected to Japan’s wartime secret military units, which are known to have experimented on humans, dead and alive, to develop chemical and other arms for mass destruction, must have been involved in the Teigin Incident because of the skill displayed in the murders and the poison’s special delayed effect.

“A former member of a special unit could have disguised himself as a health official and dealt with a special poison, as the murderer of the Teigin Incident did,” Araki said.

Hirasawa’s death sentence was finalized in 1955, and he spent 32 years on death row, during which time more than 30 justice ministers refused to sign an execution order.

His adopted son, Takehiko, 53, said at the May 10 gathering, “It has been a long struggle (for exoneration), but I want to continue it to reach our aim.”