STUTTGART, Germany — After years of relative inaction, German prosecutors have opened dozens of fresh investigations of men and women suspected of having served as Nazi death camp guards, racing against the clock to bring the aging suspects to justice. Not since the end of World War II have so many cases been initiated at once.

The surge of cases is being driven by a new generation of prosecutors — the “grandchildren generation,” as they are known here — who bring a less conflicted view of culpability to crimes committed during the war and who were given a legal opening with the 2011 conviction of John Demjanjuk, a former guard at the Sobibor death camp who spent years as an Ohio autoworker.

The prosecutors are now applying modern advances to historical crimes, like three-dimensional virtual models of camps to demonstrate what guards would have been able to see from their posts, as well as spreadsheets and databases to glean critical evidence from reams of Nazi archival material.

Since March alone, prosecutors from Stuttgart to Schwerin and Hamburg to Frankfurt have opened inquiries against men and women suspected of playing a role in the functioning of the Auschwitz-Birkenau camp, cases that could eventually lead to charges of complicity in the murder of tens of thousands of the more than one million Jews and others who perished there.

The youngest of the suspects are well into their 80s and have lived freely in Germany for decades. Among them is a woman from Hamburg who is in her 90s and served as a guard at Auschwitz-Birkenau from September to October 1944, prosecutors say. Another is a 93-year-old man who worked as a paramedic at the camp in 1944. He was arrested in March after a search of his home in the northern state of Mecklenburg-Western Pomerania.

The advanced age of the often frail suspects has brought forth sympathy among some in Germany, raising questions of whether it is just to pursue prosecutions now after having let them live out so many years in peace. But the prevailing sentiment in the German news media and public discussions is that Nazi-era crimes are better pursued late than never.

“It is often asserted, and is true, and has been taken for granted as something that was unavoidable, that most perpetrators of the Holocaust were never held accountable for their crimes or even called before a court,” said Paul A. Shapiro, director of the Mandel Center for Advanced Holocaust Studies at the United States Holocaust Memorial Museum in Washington.

“The failure to bring more Holocaust perpetrators to justice was not unavoidable,” he added, “but at the time the law was not equipped to deal with crimes committed on such a monumental scale. It has taken decades of hard work to develop the law and legal precedent necessary to fix this.”

The crucial precedent was provided by the Demjanjuk case, in which a Munich state court convicted the former death camp guard of accessory to the murder of all 28,060 people who died during the time he served there. That ruling overturned a precedent that had required evidence linking suspects to a specific killing, which had stood since 1969.

The judge further found that it was impossible for anyone who worked at Sobibor, a relatively small camp, not to be considered a part of the Nazis’ machinery of mass murder.

Since then, German federal prosecutors tasked with investigating Nazi war crimes have reopened dozens of files on former guards whose whereabouts had long been known to the authorities. The cases steadily trickled through Germany’s decentralized justice system until September, when the federal prosecutors recommended that the authorities in 11 of Germany’s 16 states pursue charges against 30 former guards.

But while the Demjanjuk ruling removed the burden of linking guards to a specific death, prosecutors continue to face the burden of proving what suspects knew about the industrial-scale killing taking place around them.

They started with Auschwitz-Birkenau, where proving what any one guard knew about the killings may be legally more difficult than in Mr. Demjanjuk’s case because Auschwitz was far larger than Sobibor.

“At a death camp like Sobibor it was clear that anyone arriving there would not be leaving again,” said Doris Möller-Scheu, a spokeswoman for prosecutors in Frankfurt who are investigating two men, identified only by their ages, 89 and 92. “We have to prove they actually served on the extermination sites at Auschwitz.”

The first case to reach state prosecutors involved Hans Lipschis, 94, who was deported from the United States in 1983. At the time, the West German authorities said they had no grounds to prosecute him. Three decades later, Ralf Dietrich, a 37-year-old state prosecutor in Stuttgart, was asked to build a case against him.

Mr. Dietrich said that while he reviewed the detailed and emotional testimony from the Frankfurt Auschwitz trials of the 1960s, it was not sufficient to build a modern case. Instead, he approached his case as he would a more recent criminal prosecution.

Using digital technology, he created a three-dimensional model of Auschwitz-Birkenau, with the help of officials in Baden-Württemberg and Bavaria, that allows for a 360-degree view of the camp. It can be programmed to show what an individual guard could have seen from a specific vantage point, even including the window of the kitchen where Mr. Lipschis said he served as a cook.

“We had to prove more than just that he was present,” Mr. Dietrich said.

“Many former guards say that they didn’t know anything because they couldn’t see from where they were serving,” Mr. Dietrich added. “This allows us to go in and look at whether that is true. What could one see from a watchtower? Could you see the chimneys of the crematoria? Could you see smoke?”

Archival research, he said, was also important. He used spreadsheets to log data from Nazi record books and officers’ diaries to calculate more precise estimates of the numbers of victims. Specifically, Mr. Dietrich drew from the meticulous diary kept by Mr. Lipschis’ unit commander to establish the exact times when the suspect served guard duty.

Armed with those dates, Mr. Dietrich turned to transport lists of prisoners who arrived at Auschwitz-Birkenau to find out how many people would have arrived under Mr. Lipschis’ watch. By subtracting the names of any known survivors, he was able to establish a plausible number of victims — 10,510 — who were killed on Mr. Lipschis’ watch, providing a legal link to charge Mr. Lipschis as an accessory to the murders.

The one obstacle Mr. Dietrich could not overcome, however, was the advanced age of the defendant, who was ruled to have the onset of dementia, rendering him incapable of following the “difficult individual charges” against him. On Feb. 28, the state court in the German city of Ellwangen dismissed the case. The ruling was a measure of the closing window of time prosecutors face to pursue their cases against the aging suspects.

While Mr. Dietrich will never know if his model would have helped convict Mr. Lipschis, his techniques are serving as a blueprint for other prosecutors across the country who are sharing information, swapping notes and trading tips on strategy.

“Of course we exchange ideas and try to help and support each other,” said Stefan Urbanek, 52, a prosecutor in Schwerin, where the 93-year-old paramedic was arrested on suspicion of accessory to the murders of more than 1,700 people at Auschwitz.

Such an approach reflects the generational shift in Germany’s justice system, which for decades seemed unwilling to seek new ways to prosecute former guards, said Nathan Stoltzfus, a professor of history at Florida State University. “It shows what you can do with a concerted effort to get a conviction,” he said.

nytimes.com