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BERLIN — A 94-year-old former guard at Auschwitz goes on trial in Germany on Thursday accused of being an accessory to the murder of at least 170,000 people — the first of four such court cases that could be the last due to the very old age of the defendants.

The three men and one woman accused are all in their nineties and will be tried over the next few months, starting with Reinhold Hanning in the western German city of Detmold.

Hanning was 20 years old in 1942 when he started serving as a guard at the Auschwitz death camp in occupied Poland where more than 1.1 million Jews were killed by the Nazis.

Prosecutors said he voluntarily joined the armed SS at the age of 18 and participated in battles in eastern Europe during the early stages of WWII before being transferred to Auschwitz in January 1942.

Accused by the prosecutor's office in Dortmund as well as by 38 joint plaintiffs from Hungary, Israel, Canada, Great Britain, the United States and Germany, Hanning will face the accounts of contemporary camp witnesses.

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One of them is Erna de Vries, who in 1943 at the age of 23 was deported to Auschwitz along with her mother. Considered a "Jewish crossbreed" as her father was Protestant, she was saved from the gas chamber and transferred to a labor camp.

Related: Elderly Nazi SS Suspects Raided in 1944 Massacre Probe

"I survived, but up until today I don't know how exactly my mother was killed," de Vries told Reuters ahead of the trial. "The last thing she said to me was, 'You will survive and tell what happened to us.'

"I am not hateful but it somehow feels like justice to see this man, who was working there when my mother died, on trial," die Vries added.

The entrance of the former German Nazi death camp of Auschwitz. Czarek Sokolowski / AP

Investigations by Germany's special Nazi war crimes office in Ludwigsburg show that Hanning served as a guard at Auschwitz until at least June 1944.

Related: 95-Year-Old Will Be Tried on Auschwitz Allegations

While Hanning admitted to his guard duties in a statement to the prosecution, he denied involvement in the mass killings. But investigators say he also served at Auschwitz's Birkenau sub-division where about 90 percent of more than 1.2 million killings in the camp were carried out in four gas chambers.

Prosecutors maintain that the Nazis' killing machinery hinged on people like Hanning guarding the prisoners and accuse him of expediting, or at least facilitating, the murders.

Precedence for such charges was set in 2011 when death camp guard Ivan Demjanjuk was convicted of being an accessory to mass murder.

Given the age of the accused, trials are delayed due to lengthy procedures to determine whether they are fit to be in court. Hearings are also restricted to two hours per day.