Dec 13 marked the 81st anniversary of the Nanjing Massacre. But that’s just one of the atrocities committed by the Japanese Army during World War Two. One North-American filmmaker is trying to shed light on another – and how the United States let the perpetrators escape justice. CGTN’s Mark Niu reports.

At the office of San Francisco’s Pacific Atrocities Education, a new documentary – Unit 731: How America Exploited Japan’s Biological Weapons Crimes – is being screened for the first time in the United States.

It was five years ago while working in China that journalist Paul Johnson first heard about the Japanese Imperial Army’s Unit 731 and its biological experiments on thousands of Chinese civilians.

“I was outraged morally that this was something that had happened and we don’t know about this,” said filmmaker Johnson. “This isn’t taught like other events that are similar in barbarity are taught. So I started investigating.”

Johnson also traveled throughout China to find victims who believe they were infected by the Japanese Imperial Army’s germ bombs.

To this day, they have what is known as rotten leg syndrome – a condition where open wounds never heal.

“Is this history going to disappear as the people who commit it disappear, as the survivors who were in that event disappear? And how do we hold on to that memory?” asked Nikki Arcado, a researcher at the Pacific Atrocities Education non-profit group.

In his film, Johnson tracks down a member of Unit 731 who is still conflicted about the barbaric experiments he conducted on Chinese civilians and his own duty to serve the Japanese military.

But the film unravels more layers, by delving into how the United States failed to go after those responsible for the crimes.

“The US literally made a deal with the devil. They knew who the perpetrators were, they knew who the leader was – Shiro Ishi. In fact, they had him in Tokyo. But word came from Washington: we’ll give you immunity from prosecution in exchange for the data that you developed of human experimentation.”

His documentary raises even more controversial claims about the US’s role.

A researcher in his film believes it’s highly likely that the US used the information gained from Unit 731 to conduct its own biological tests that were used during the Korean War.

While that remains shrouded in secrecy, Johnson says the US’s failure to seek justice for war crimes is clear.

“I think an apology is owed on behalf of Washington DC and the American people, who would be appalled mostly to know about this to the people of China. This was a very sad, dark moment in US history. And we should atone for it. As an American, I believe that’s something we should do and could only help our relationship with China.”

Johnson also leaves viewers questioning whether biological weapons development in a number of Western countries could all be traced back to the notorious legacy of Unit 731.