The last living Nuremberg Trials prosecutor has issued a chilling reminder about the horrors of war.

Ben Ferencz, now aged 97, helped bring to justice 22 Nazi SS officers who were behind the murder of more than one million people.

The officers Mr Ferencz prosecuted were part of Nazi units called Einsatzgruppen (“action groups”) that were tasked with following the German army as it invaded the Soviet Union in 1941 and killing any Jews, gypsies and Communists they encountered.

In total, they are believed to have killed more than one million people in cities, towns and villages across Eastern Europe.

Mr Ferencz, who moved to the United States from Romania with his family when he was a baby, was educated at Harvard Law School.

During the Second World War he enlisted as a private in the US army and was soon transferred to a new unit that was tasked with investigating Nazi war crimes.

He was one of the first people on the scene at the newly-liberated concentration camps.

Speaking to CBS News, he told the story of meeting a son at a concentration camp whose father had died shortly before the camp was liberated.

The son told him how his father had saved a small piece of bread for his child every day and kept it under his arm at night so other inmates would not steal it.

“[I heard] these human stories which are not real”, Mr Ferencz said. “But they were real.”

After the war, Mr Ferencz initially returned home but was soon asked to lead a team of researchers looking into Nazi war crimes with the aim of identifying people to prosecute.

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One of his team discovered a batch of secret documents that laid bare the brutality the Nazis had inflicted, not just inside concentration camps. There were reports from the Einsatzgruppen operating in Eastern Europe, documenting how they had gunned down thousands upon thousands of Jews, gypsies and others.

“These were daily reports from the Eastern Front: which unit entered which town, how many people they killed”, Mr Ferencz said. “It was classified; so many Jews, so many gypsies, so many others.”

“They were 3,000 SS officers trained for the purpose, and directed to kill without pity or remorse, every single Jewish man, woman, and child they could lay their hands on.”

Their reports were predictably chilling. Included in them were phrases such as “In the last ten weeks, we have liquidated around 55,000 Jews” and, in one memo from Kiev in 1941: "The city's Jews were ordered to present themselves… about 34,000 reported, including women and children. After they had been made to give up their clothing and valuables, all of them were killed, which took several days."

Mr Ferencz began adding up the number of people killed and soon realised the scale of the massacres.

“When I reached over a million people murdered that way, over a million people, that's more people than you've ever seen in your life, I took a sample”, he said.

“I got on the next plane, flew from Berlin down to Nuremberg, and I said to [General Taylor, who was leading the trials] "General, we've gotta put on a new trial."

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Mr Ferencz was just 27 at the time and said he had not even set foot in a court room before. Yet the Nuremberg prosecutors were so stretched that General Taylor told him that, if he wanted to bring members of the Einsatzgruppen to justice, he would have to do it himself.

His work resulted in 22 Nazi officers being convicted and four of them hanged.

Reflecting on his experiences, Mr Ferencz issued an important message about war. He said the Nazi soldiers who committed atrocities were not “savages” but “intelligent, patriotic human being[s]”, and that war can make any normal person do horrifying things.

“Do you think the man who dropped the nuclear bomb on Hiroshima was a savage?”, he asked.