It is clear that Japan will never hang Sadamichi Hirasawa. He has been living on Death Row for more than 30 years (that is said to be a world record), but the executioner has never called. Hirasawa is now 93 and a near-blind invalid. When the authorities talk about Hirasawa, which is rare, they often cite his infirmity, hinting that sending a man to die in his condition would offend public sensibilities.

Japan's criminal-justice system has an almost four-decade investment in Hirasawa as the man behind what Japanese call the Imperial Bank Incident, the Jan. 26, 1948, poisoning of 12 people, which ranks among the most fiendishly planned and executed mass murders in history. Official consideration of his case becomes bureaucratically harder each year.

The old man continues to turn up in Japanese newspapers several times a year, when his support group on the outside, the Committee to Save Hirasawa, makes a new filing in court or his health takes another turn for the worse. Authorities have cut off most of his visitors.

William Triplett, a 28-year-old Washington journalist and playwright who became interested in the case through a newspaper article one Sunday morning, has turned up the first genuinely new material on it in years. His evidence tends to strengthen the longstanding committee thesis, which at first seems absurdly conspiratorial but after study more plausible than the courts' verdict: That the real culprit was a veteran of a Japanese Imperial Army germ warfare unit during World War II, that after the war he was under the protection of U.S. occupation authorities and that the police, desperate to salvage their standing with the public, railroaded Hirasawa.

The book reveals "the startling truth" about the case, the dust jacket informs us. Inside, we look in vain for a smoking gun. Triplett concludes that "exactly who committed the murders will probably remain a point of conjecture." Triplett's prose generally flows pleasingly but hops annoyingly between the tone of a spy thriller and that of a pop anthropology text. We keep reading, though. The facts are riveting.

Wearing a government armband and brown suit, a gray-haired man appeared just after closing time at a Tokyo branch of the Imperial Bank on Jan. 26, 1948. He explained that he was a doctor and that a man with dysentery had called at the bank earlier in the day. It was necessary to disinfect the premises immediately and treat everyone with a special drug, he said. Americans would soon be along to check.

Sixteen employes dutifully lined up and, under the man's reassuring but firm supervision, bolted doses of a liquid he had dispensed into teacups. It was a form of cyanide. While his victims writhed on the floor, the man took a check and about 180,000 yen in cash and disappeared. Only four of the 16 survived.

From that day on, the honor of the entire Japanese police force, at that time being rehabilitated under close American supervision, was at stake. The biggest manhunt in Japanese history was launched, reaching into virtually every household in the country. Some gray-haired men began dyeing their hair to avoid the constant questioning by police.

Six months later, police arrested Hirasawa, a professional tempera painter with a history of petty fraud, a stash of money he could not explain and a chain of circumstantial links to the crime. After 62 sessions with interrogators, he confessed in full. But he recanted later and went to trial. He was convicted and, his appeals exhausted, entered Death Row in 1955.

He had been there two decades when one of the occupation's darkest secrets became public knowledge through the press. U.S. authorities after the war had granted immunity from war crimes prosecution to former members of an Imperial Army group known as Unit 731. It was given in exchange for results of ghastly experiments 731 had conducted on Chinese, Soviet and a few American prisoners of war, including germ infection, human vivisection and, significantly, cyanide poisoning.

Triplett obtained occupation-era U.S. documents through the Freedom of Information Act that show clearly that in the months after the crime, the Japanese police's prime theory was that the murder was the work of a 731 man.

The equipment, the poison, the procedure, even the English-language labels on the jars the murderer had carried matched nearly perfectly with ones developed by 731. More importantly, perhaps, was that the murderer seemed to have had experience, both in preparation of poison and the macabre psychology of making people cooperate in their own murder.

But the paper trail abruptly runs out. To buy the argument of the Committee to Save Hirasawa, we must make a leap on our own, that U.S. authorities called a halt to the 731 lead out of fear it would prove true, and any resulting trial would expose the immunity deal. (The Cold War was well under way by this point.) The police had to have someone and Hirasawa blundered along.

Triplett's documents, in fact, were submitted to a Japanese court this year to support another attempt by the committee to free the man. The group attempted to invoke a clause in Japanese law that nullifies a death sentence if the offender remains at large for 30 years. No, the court said, Hirasawa does not qualify, despite his time under sentence, as he has been in jail, not on the outside.