The bones, stored in a repository for decades, are a subject of debate. Some believe they are remnants from early 20th-century anatomy classes, while others argue they represent the unidentified remains of victims of Japan’s notorious war crimes.

A group of activists, historians, and experts advocating for a government investigation into wartime human germ warfare experiments gathered over the weekend to mark the 35th anniversary of the bones’ discovery. They renewed their call for an independent panel to examine the evidence.

Japan’s government has historically avoided discussing wartime atrocities, including the sexual abuse of Asian women known as “comfort women” and the forced labor of Koreans in Japanese mines and factories, often citing a lack of documentary proof. While Japan has apologized for its aggression in Asia, it has faced repeated criticism from South Korea and China since the 2010s for backtracking on its commitments.

Around a dozen skulls, many bearing cuts, and fragments of other skeletons were unearthed on July 22, 1989, during the construction of a Health Ministry research institute on the former site of the wartime Army Medical School. The school’s close connections to a germ and biological warfare unit have led many to believe that the remains could be linked to a dark chapter of history that the Japanese government has never officially acknowledged.

Headquartered in then-Japanese-controlled northeast China, Unit 731 and associated units conducted experiments on prisoners of war, injecting them with typhus, cholera, and other diseases, according to historians and former unit members. They also allege that the unit performed unnecessary amputations and organ removals on living subjects for surgical practice and subjected prisoners to freezing experiments to test their endurance. The Japanese government has only acknowledged the existence of Unit 731.

Top officials from Unit 731 were not tried in postwar tribunals, as the U.S. sought access to their chemical warfare data, historians say. However, lower-ranking officials were tried by Soviet tribunals. Some of the unit’s leaders went on to become medical professors and pharmaceutical executives after the war.

A previous Health Ministry investigation, conducted in 2001, concluded that the bones could not be linked to Unit 731, suggesting they likely originated from bodies used in medical education or brought back from war zones for analysis. This conclusion was based on interviews with 290 individuals associated with the school.

The report acknowledges that some interviewees drew connections to Unit 731. One witness claimed to have seen a head in a barrel shipped from Manchuria, where the unit was based. Two others mentioned hearing about specimens from the unit being stored in a school building, but had not personally seen them. Others denied the link, suggesting the specimens could have originated from the prewar era.

A 1992 anthropological analysis determined that the bones belonged to at least 62 and possibly more than 100 different individuals, primarily adults from parts of Asia outside Japan. The analysis noted that the holes and cuts found on some skulls were made after death but did not find evidence linking the bones to Unit 731.

Activists, however, maintain that the government could do more to uncover the truth, including publishing complete accounts of its interviews and conducting DNA testing.

Kazuyuki Kawamura, a former Shinjuku district assembly member who has dedicated much of his career to resolving the bone mystery, recently obtained 400 pages of research materials from the 2001 report through freedom of information requests. He contends that the government “tactfully excluded” key information from witness accounts.

While the newly released materials do not contain conclusive evidence, they include vivid descriptions—the man who described seeing a head in a barrel also detailed handling it and subsequently running off to vomit—and comments from several witnesses suggesting that further forensic investigation might reveal a connection to Unit 731.

“Our goal is to identify the bones and send them back to their families,” stated Kawamura. He believes the bones are virtually the only proof of what happened. “We just want to find the truth.”

Atsushi Akiyama, a Health Ministry official, asserted that witness accounts had already been analyzed and incorporated into the 2001 report, and the government’s stance remains unchanged. He highlighted the absence of documentary evidence, such as labels on specimen containers or official records, as a key missing link.

Documents, particularly those involving Japan’s military, were meticulously destroyed during the war’s final days, making it difficult to find new evidence for proof.

Akiyama added that the lack of information about the bones would make DNA analysis challenging.

Hideo Shimizu, who was sent to Unit 731 in April 1945 at the age of 14 as a lab technician, participated in the meeting online from his home in Nagano. He recalled seeing heads and body parts preserved in formalin jars stored in a specimen room in the unit’s main building. One particular sight that haunted him was a dissected belly containing a fetus. He was told they were “maruta”—logs—a term used for prisoners chosen for experiments.

Days before Japan’s surrender on August 15, 1945, Shimizu was instructed to collect the bones of prisoners whose bodies were burned in a pit. He was then given a pistol and a packet of cyanide to take his own life if captured on his journey back to Japan.

He was ordered to never speak about his experiences at Unit 731, to never contact his colleagues, and to never seek a government or medical job.

Shimizu stated that he cannot tell if any of the specimens he saw at Unit 731 are among the Shinjuku bones based on their photographs, but that the horrors he witnessed in Harbin should never be repeated. When he looks at his great-grandchildren, he says, they remind him of the fetus he saw and the lives lost.

“I want younger people to understand the tragedy of war,” he said.