MAINZ, Germany -- In their search for justice that has endured for decades, the biggest challenge Nazi hunters face is time. 

The knowledge that war criminals are escaping prosecution through death by natural causes means their task has never been more pressing.

On Monday, German state police arrested a 93-year-old man accused of being a guard at the Auschwitz concentration camp. Hans Lipschis is the first suspect to be facing charges as part of a drive launched earlier this year to track down 50 suspected Auschwitz guards who are believed to be living in Germany.

Most of those involved in the murder of about 6 million Jews in the Holocaust and still alive will now be in their 90s, a ripe old age for people who carried out one the most heinous crimes in the history of humanity.

But that doesn't stop Kurt Schrimm, director of Germany’s Central Investigation Center for Nazi Crimes. His agency employs 20 people, including seven focusing on the Auschwitz cases. 

"Someday there will be no more Nazi criminals to go after and then our organization will shut down," he said. "But until then, we will exhaust all investigation possibilities."

After years of frustration, Nazi hunters have also been given fresh hope by a German court's landmark ruling that has made it simpler to prosecute cases by opening the door to charges of "accessory to murder."

Efraim Zuroff, Israel director of the Simon Wiesenthal Center in Jerusalem, said he planned to ask German companies to help fund a renewed campaign to find the remaining war criminals and take advantage of the ruling, which came during the successful prosecution of John Demjanjuk.

Demjanjuk, an autoworker who lived in the U.S. for years after the war, was convicted in 2011 of 28,060 counts of being an accessory to murder and sentenced to five years in prison.

Although he died a free man in a nursing home in Germany – he was released pending his appeal – the court’s ruling that he could be convicted on his service record alone was “a total game-changer,” Zuroff said.

“Until that point … German prosecutors could not try a case unless they had evidence of a specific crime with a specific victim,” he said.

“Demjanjuk was convicted solely for his service as an armed SS guard at a death camp,” he added. “As a result, this opened up a whole new potential number of people to bring to justice.”

Zuroff said there were usually three obstacles to holding Nazi war criminals to account: Finding them; getting enough evidence; and persuading the authorities to act.

The Demjanjuk ruling changed that in Germany.

“Now in Germany, all of a sudden, all you have is one task – all you have to do is find people, because you can prove service with documents,” Zuroff said. “You don’t have to have someone who says, ‘I saw this bastard kill my fellow inmate.’”

Schrimm said that the Demjanjuk case prompted his agency to start "looking at old files with a renewed focus."

He added: “Today, any job in a concentration camp can be sufficient evidence towards a conviction as accessory to murder."

It is a ray of hope in an otherwise gloomy picture. 

“Once the Nuremberg Trialshad been completed [in 1949], the prosecution of Nazi war criminals never became a serious priority in any country outside of the Soviet Union,” Zuroff added. “The failure to do more to hold the perpetrators of the Holocaust accountable is naturally a source of frustration and disappointment for me personally, as someone who has devoted practically my entire adult life to that mission."

The Holocaust saw approximately 6 million Jews – about two-thirds of the pre-World War II Jewish population in Europe – murdered to fulfill Adolf Hitler’s infamous “Final Solution.”

Roma Gypsies, Slavic people such as Poles and Russians, communists, socialists, disabled people, Jehovah’s Witnesses, homosexuals and others were also slaughtered in large numbers.

Zuroff said that no one really knew how many people were involved in the killings, let alone how many were still alive.

But, asked to estimate, he reckoned that “probably not more than 10 to 15 percent” of tens of thousands of Nazi war criminals had been brought to justice.

The Simon Wiesenthal Center publishes an annual “most wanted” list, and also rates countries based on their willingness to take action. Only the United States got the top rating in 2013; Germany was among five countries in the second-highest group.

Zuroff said that “to their credit” Germany was one of the few countries that would bring prosecutions.

In contrast Austria, which became part of the Hitler’s Third Reich in 1938, was “horrific, terrible, the worst,” Zuroff said.

“They haven’t succeeded in taking action against a Nazi war criminal in more than 30 years. It’s not because there are no Nazis in Austria,” he said. “There’s a country that until 20 years ago … got away with claiming they were Hitler’s first victim. Austrians played a very leading role in the murders carried out by the Third Reich.”

Zuroff said it was “impossible” to get prosecutions in the Baltic countries, “especially in Lithuania.”

“They were the worst because they had a vast number of collaborators,” he said. “They don’t like punishing their own people and would prefer to think of themselves as victims of communism and not killers of Jews, which they were. They were outstanding killers of Jews.”

Avner Shalev, chairman of Yad Vashem, Israel’s official memorial to victims of the Holocaust, said the survivors "live with the memories every day."

"Bringing the perpetrators to justice sends an important educational and moral message to society at large: These kinds of crimes will not be tolerated, and there are no free passes," he said. "Although unfortunately many of the perpetrators escaped justice, nevertheless each trial sends an important message."

Germany and its allies controlled most of Europe during World War II, including Norway, France, Italy, Greece, the Balkans, Poland and deep into the then Soviet Union.

Lydia Brenners was just 9 years old when she was caught up in a horrific massacre of Jews in Novi Sad in modern-day Serbia by Hungarian forces in 1942. Nazi-allied Hungary had annexed the area in 1941.

Brenners said she was forced to go to with her father, mother and sister to a local theater where many Jewish people were being gathered. They were taken in groups to the banks of the River Danube, where they were shot dead. A total of more than 1,200 civilians are thought to have been killed, according to The Associated Press.

“Slowly we came nearer and nearer [to the end of the line],” said Brenners, now 81 and living in Rishon Letzion, Israel. “Today I know it was for killing. Then … I didn’t know, maybe the older people understood.”

“In the row behind me, there was an auntie of one of my girlfriends. I knew her. She was holding a baby in her hands,” she said. “After a few minutes … [she] burst out with nerves and started to shout, ‘I cannot bear it anymore.’”

“The soldiers came and took her,” she said, despite efforts of others who surrounded her in an unsuccessful attempt to save her. “She did not come back from there.”

But then came an order from Budapest to stop the killing and Brenners and her family were released. They then took the train to Budapest that day and hid in the city until it was taken by Soviet troops toward the end of the war.

Brenners said years later she met a woman who said she was the child of her friend’s aunt. The woman was still trying to find out how she survived.

Brenners said she remembered an officer on a horse -- who was addressed as “Shanny” -- overseeing the massacre and the gendarmes referring to lists of names when deciding who should be taken.

She said “Shanny” was a nickname for Sandor Kepiro, a gendarme officer accused of helping organize the killings.

Kepiro was given a 10-year prison sentence over the Novi Sad massacre by a Hungarian court in 1944, but this was overturned after Germany formally occupied Hungary later that year, according to The Associated Press.

Kepiro, who lived in Argentina after the war, admitted he was present and supervised the identities of those being rounded up, but denied knowing they were killed until later, the news service said.

Kepiro was tried again in Hungary but acquitted in 2011, with a court ruling there was insufficient evidence against him, the AP reported. The prosecution appealed, saying the judges’ decision was “unfounded,” and so did the defense, which complained the ruling had not actually cleared Kepiro.

However, Kepiro died in September 2011, an innocent man in the eyes of the law, a hero to some in Hungary, but a killer who escaped justice to Zuroff and his fellow Nazi hunters.

Ian Johnston reported from London. The Associated Press contributed to this report.