As the prisoners disembarked from the trains that took them to the Auschwitz-Birkenau death camp in Nazi-occupied Poland, a young, low-ranking S.S. guard named Oskar Groening waited for them on the platform.

He was an accountant of sorts. Tasked with retrieving their luggage and belongings, Groening also confiscated their money, recorded the amounts in a ledger and then sent it to headquarters in Berlin.

Seven decades after the liberation of the camp, Groening—then a frail, 93-year-old man—was charged and convicted by a court in Luneburg, Germany, for being an accessory in the murder of 300,000 mostly Hungarian Jews from May to July 1944, and sentenced to four years in prison.

His 2015 prosecution and the recent efforts to bring other aging former Nazis to justice is the subject of the Canadian documentary “Accountant of Auschwitz,” set to debut Sunday at the Hot Docs Canadian International Documentary Festival.

In the aftermath of the Second World War, Germany was reluctant to prosecute former Nazis, which meant the vast majority of them were never prosecuted.

The country was in ruins, and the lawyers and judges who would have been responsible for initiating these criminal proceedings were often former Nazis themselves, Ricki Gurwitz, the documentary’s producer, told CTV News Channel.

“They were sitting in judgement of themselves,” she said. “The ones they did prosecute were the most sadistic, the ones who poured Zyklon B into the gas chambers.”

A narrow legal framework for trying Nazis required prosecutors to prove both that the accused was directly involved in the killing of a specific victim, and that the killing was motivated by racial hatred.

But in 2009, the framework changed. Suddenly, just being present at a death camp could be grounds for prosecution.

The problem was that many Nazis were dead or too sick to stand trial. Groening was alive and considered to be healthy enough.

“In previous trials, most Nazis said that they were just following orders or they just sat there and didn’t speak,” Matt Shoychet, the documentary’s director told CTV’s Your Morning.

Groening was different.

“He said, ‘I was there. I saw the killing pits. I was there,’” Shoychet said.

Some wondered whether such an elderly man should have been brought to trial in the first place. But the judge disagreed, finding that, while there was no evidence that Groening actually killed anyone, he knew what was happening and his actions indicated complicity.

Groening died in March 2018, before he served any of his prison sentence.

Gurwitz said that the documentary holds contemporary lessons.

“The Holocaust is fading from memory and from textbooks in schools,” she said. “When you see what’s going on in Syria and what’s going on around the world, the slogan of ‘never again’ is almost meaningless.”