Seiichi Morimura, who wrote a searing exposé of the Japanese army’s secret biological warfare program in occupied China, describing how it forcibly infected thousands of prisoners with deadly pathogens, died July 24 in Tokyo. He was 90.

The announcement of his death by his publisher, Kadokawa, was cited in Japanese media.

Morimura detailed the atrocities committed by the Japanese program — called Unit 731 — in a widely sold book, “Akuma no Hoshoku,” or “The Devil’s Gluttony” (1981). Among the horrors he described were vivisections performed without anesthesia on those who had been deliberately administered germs; doctors wanted to see firsthand how the ensuing diseases infected the body.

Under the Japanese occupation, before and during World War II, at least 3,000 prisoners — men, women and children — became guinea pigs at a facility euphemistically named the 731st Epidemic Prevention and Water Supply Headquarters, on the Manchurian plain near Harbin. Most of the victims were Chinese, but some were Korean, Russian and Mongolian.

All are believed to have died from the torture.

In addition to those exposed to pathogens — including plague, typhus, cholera, syphilis and anthrax — some men were subjected, naked, to freezing temperatures for long periods; their frozen flesh and limbs were then pounded with boards to measure their sensitivity.

Others, Morimura wrote, underwent transfusions with horses’ blood. Some were exposed to X-rays for prolonged periods. Some were locked in a pressure chamber to see how long it took before their eyes popped out of their sockets. Still others were tied to stakes while a canister of a pathogen was exploded nearby.

The unit also developed germ bombs that were tested in Chinese cities, reportedly killing at least 200,000 people. In at least one case, planes dropped plague-infested fleas over Ningbo, in eastern China.

The unit had proposed sending balloon-borne disease bombs to the United States as well, Morimura found. He believed that they would have been used if not for the atomic bombing of Hiroshima and Nagasaki that ended the war in the Pacific.

Morimura, a mystery novelist and a pacifist, had mentioned Unit 731 in a novel and was contacted by one of its workers, moving him to investigate its brutalities. He first wrote about the unit in a series of articles for a communist newspaper in Japan.

He said the goal of his book was to bring balance to Japanese accounts of the war.

“Almost all Japanese war themes are from the standpoint of Japan as a victim,” he told The Age, a newspaper in Melbourne, Australia, in 1982, “but mine is from the point of view of Japan the transgressor doing violence against other nations.”

Morimura’s book sold more than 1.1 million copies within seven months of its publication. It was not the first account of Unit 731’s brutality — there were two others in the 1960s and ’70s — but Morimura’s was drawn from interviews with 60 Japanese participants in the program.

“Mr. Morimura’s 246-page book is believed to be more accurate and more believable” than the others, The New York Times reported in 1982. The article quoted Morimura as saying: “This story should be told to all Japanese, to every generation. Japanese aggression should be written about to prevent another war.”

The book prompted grudging acknowledgment of Unit 731’s atrocities by a Japanese government official, who told the country’s parliament in 1982 that the grisly experiments had occurred “during the most extraordinary wartime conditions” and were “most regrettable from the viewpoint of humanity.”

Separately, an article in the journal The Bulletin of the Atomic Scientists reported in 1981 that officials behind Unit 731 — including its leader, Lt. General Shiro Ishii, who remained free until his death in 1959 — were granted immunity from prosecution as war criminals by the United States in exchange for Americans’ securing “exclusive possession of Japan’s expertise in using germs as lethal weapons.”

Seiichi Morimura was born Jan. 2, 1933, in Saitama prefecture, north of Tokyo. He survived the United States bombing of Tokyo near the end of World War II. According to his publisher, he graduated from Aoyama Gakuin University in Tokyo and worked in hotels before he turned to writing.

Information about his survivors was not immediately available.

Morimura is reported to have written about 300 books, nearly all of them mystery novels. A recent book ventured into nonfiction, however, about his commitment to defending Japan’s postwar pacifist constitution against Prime Minister Shinzo Abe’s campaign in 2015 to increase military activity, The Associated Press reported.

But “The Devil’s Gluttony” earned Morimura the most renown.

His role in making public the horrors of Unit 731 — and the publication of books on the subject that followed his — has reverberated for decades. In an investigative article in the Times by Nicholas Kristof in 1995, a farmer who had been a medical assistant in Unit 731 recalled dissecting a man without the use of anesthesia.

“I cut him open from the chest to the stomach, and he screamed terribly, and his face was all twisted in agony,” the man was quoted as saying. “He made this unimaginable sound, he was screaming so horribly. But then finally he stopped. This was all in a day’s work for the surgeons, but it really left an impression on me because it was my first time.”

Such revelations echoed in a lawsuit filed in 1997 by family members of some of Unit 731’s Chinese victims. A Tokyo District Court judge ruled in 2002 that the program had “used bacteriological weapons under the order of the Imperial Japanese Army’s headquarters,” but the judge rejected awarding them compensation, saying they had no right as foreign citizens to demand money from Japan under international law.

Nor did the plaintiffs have support from China, which prevented them from organizing, or the United States, which was wary of alienating Japan, a staunch ally.

“We are fighting Japan, China and the United States all at once,” Wang Xuan, one of the plaintiffs, told the Times after the decision. “We need endless amounts of time to do this, and time is running out.”

Three years later, the Tokyo High Court upheld the decision denying compensation.

But thanks to Morimura’s work and that of others, the Japanese authorities have increasingly acknowledged the enormity of Unit 731’s horrors. In 2018, Japan’s national archives released the names of 3,607 people — among them medics, doctors, surgeons, nurses and engineers — who had been members of the program.

And at a Unit 731 museum, in a suburb of Harbin, a sign by the exit reads: “The Unit 731 site is by far the largest historical site of biological warfare in world war history. It is also a historical witness to human suffering, a legacy and special memory of a brutal war.”

This article originally appeared in The New York Times.