Trials of former Nazi concentration camp guards in Germany have become rare in recent decades: As more and more of the perpetrators have died, prosecutors find it increasingly hard to charge those responsible for the horrendous crimes.

The current trial of 94-year-old Reinhold Hanning, a former SS guard at Auschwitz, may be one of the last of its kind. Hanning is accused of participating in mass shootings and selecting inmates for the executions. The trial started Thursday.

According to the prosecutor's office, Hanning may have been involved in the killing of at least 170,000 people, most of them Jews. More than 1 million people died in Auschwitz alone during World War II. During the time he was a guard at Auschwitz, hundreds of thousands of Hungarian Jews were killed by the Nazis.

According to German news site Spiegel Online, survivors of Nazi concentration camps were among those attending the beginning of the trial in the city of Detmold on Thursday. Several are expected to testify in coming weeks.

One of them, 94-year-old Leon Schwarzbaum, described the ordeal, the Associated Press reported: "The chimneys were spewing fire ... the smell of burning human flesh was so unbelievable that one could hardly bear it."

Hanning denies the charges but acknowledges that he worked in the camp as a guard.

Within the next months, two other men and one woman who are also accused of having been Nazi guards in concentration camps are expected to go on trial in Germany.

For decades, prosecution of Nazi crimes focused on high-level officials and generals. Partially due to a lack of evidence but also given the large number of low-level perpetrators, prosecutors rarely investigated crimes committed by camp guards. That, however, changed after John Demjanjuk, a retired U.S. autoworker, was convicted on more than 28,000 counts of accessory to murder in 2011.

Only a handful of suspects have stood trial in recent years because it has become increasingly difficult to find evidence of direct involvement in the mass killings. Many others have died before they could be charged.

The German newspaper Frankfurter Allgemeine Zeitung reminded its readers that the chances Nazi guards would be sentenced for their crimes had increased since a verdict last year.

Previously, courts had sentenced perpetrators only if they had worked at sites that were exclusively used as death camps, such as Treblinka and Sobibor. Auschwitz was not considered such a camp, which helped many Nazi guards avoid going to jail. Last year, however, a court sentenced an Auschwitz guard to four years in jail for having helped to murder 300,000 people.