Fifteen years have passed since human bones were dug up at a construction site in Shinjuku Ward, Tokyo, linked to the infamous wartime Unit 731, and they remain a mystery that authorities still appear reluctant to resolve.

The Group to Investigate the Bones Issue, made up of local residents and scholars, suspects many of the deceased were victims of wartime experiments carried out on prisoners by the notorious Imperial army unit and is calling on the government to use DNA and superimposing technology to examine the remains to verify their origins.

The bones were dug up in July 1989 adjacent to Toyama Park at a site near dozens of low-cost public housing units.

From the 1920s through 1945, the army operated a medical school and an epidemic prevention institute at the site. The facilities had connections with Unit 731, which is believed to have waged germ warfare activities in Manchuria during the war as well as carried out experiments on humans, including live vivisections.

After the war, the buildings were used by government institutions studying infectious diseases and nutritional issues until they were demolished in the 1980s.

Today, the National Institute of Infectious Diseases and the National Institute of Health and Nutrition sit on the site.

The citizens’ group has urged the government to confirm the identities of the deceased as part of Japan’s war atonement.

Kanagawa University professor Keiichi Tsuneishi, who heads the group, said Japan’s failure to address the issue over the past 15 years shows how the people disregard Unit 731’s atrocities.

“The government has not properly cared for the victims of the war,” Tsuneishi said. “Even after the bones — which are physical (evidence) that may be connected to Japan’s wartime deeds — were unearthed, the government has acted in an irresponsible manner in dealing with them.”

Right after the bones were unearthed, a special scientific investigative unit of the Metropolitan Police Department examined some of the bones and determined the deceased had perished more than 20 years earlier.

According to a 1992 study by then Sapporo Gakuin University anthropologist Hajime Sakura, the bones were those of at least 62 people, and most of them were Mongoloid. In addition, many of the skulls had holes, cuts and other marks indicative of brain surgery.

Sakura performed the study at the request of the Shinjuku Ward office.

The former Health and Welfare Ministry headed the government effort to find the origin of the bones through interviews and questionnaires with 368 former army officers related to the medical school, a ministry official said.

A 2001 report by the ministry, now known as the Health, Welfare and Labor Ministry, said that based on the interviews and the survey, some of the remains were probably bodies kept as specimens at the medical school for educational purposes.

It also mentioned the possibility — without elaborating — that some of the deceased were bodies brought from battlefields.

However, the report concluded there was no solid evidence to link the remains with the activities of Unit 731, and it noted that some interviewees pointed to this possibility while others denied it.

Yoshio Shinozuka, 82, who served as a civilian in Unit 731 from 1939 and 1943, claimed he had never seen the bodies of Chinese victims of human experiments being shipped to Japan or heard of such action.

“But I think we can’t deny the possibility (that some of the bones were the remains of bodies sent from Unit 731) because the epidemic prevention institute was linked with the unit,” he said, noting that the government should get to the bottom of the mystery.

Members of the citizens’ group speculate that Unit 731’s victims’ corpses were sent to the medical school for research purposes. They also believe that the bodies sent to the school include Chinese who were used for live vivisections at other army-related institutes.

In 2002, the ministry built a charnel near the site to keep the remains so they can be available for future examination if new information that can establish the identity of the deceased comes into play. No further investigation on the remains has been carried out since.

Tsuneishi’s group hopes to change this.

“The government should conduct DNA tests on every bone to pinpoint the ethnic origin, as well as figure out the exact number of corpses involved,” said Kazuyuki Kawamura, a member of the group.

The group is demanding that the ministry try a superimposing method to identify the dead. The method can help put a name to skeletons by superimposing three-dimensional images of the skulls on photographs of people who may have been victims.

“We can try the method because photographs of some Chinese who were captured by Japanese military police (in northeastern China) and brought to Unit 731 can be found in official military police documents, which are kept in China’s public record offices,” said Kawamura, a former member of the Shinjuku Ward Assembly.

The group is also demanding that the ministry and Shinjuku Ward excavate another site near Toyama Park where it suspects, based on testimony of some surviving officials, that more remains might be buried. The authorities have refused to comply.

Tsuneishi said a law must be enacted to oblige the government to investigate the Toyama bones as part of the nation’s efforts to atone for its wartime misdeeds.

“We have seen how the bureaucracy works. Nobody takes responsibility” for unwanted tasks, he said. “If bureaucrats don’t want to do something, they won’t. So we need a law to force them to work on what they are hesitant to take up in order to solve this mystery.”

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