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"Sometimes the exposure and the revelations about what these people did are more painful to them than sitting in jail - in most cases their families don't have a clue what they did."
Between 1941 and 1945, six million Jewish men, women and children were murdered by the Nazis in barbaric death camps.
But in the weeks and months after the end of the Second World War, thousands of soldiers and officers responsible for torturing these people escaped - many going on to marry and raise families of their own.
And while the few who remain are well into their old age, one man is determined to find them - and bring them to justice.
Dr Efraim Zuroff has dedicated 40 years of his life to tracking down those who escaped - and tells LADbible he won't stop until they are exposed for the crimes they have committed.
It has, at times, been a dangerous obsession.
"I get quite a few threats on the internet, and if I lived in Europe I would definitely need protection," the 71-year-old grandfather says.
"There was one case where Croatian emigres in Australia put a price on my head, saying that if anything happened to an Ustashe Nazi War Criminal we had exposed (Georg Aschner), they would pay for me to be killed.
"They also put a bounty on two other people - for more money, which was quite embarrassing, I have to say.
"In the end we exposed the criminal they were trying to protect but he escaped to Austria. Their authorities refused to hand him over, claiming he wasn't fit to stand trial - even though he gave a brilliant interview to a newspaper during the Euros in 2008."
Born in New York in 1948 of Lithuanian descent, Zuroff is an historian at heart.
After graduating from the Yeshiva University, he moved to Israel to work at the Yad Vashem, the country's memorial museum for the victims of the Holocaust.
In 1978, he returned to the States to work at the esteemed Simon Wiesenthal Centre, named after the Austrian Holocaust survivor who tracked down more than 1,000 Nazis before his death in 2005 - including Karl Silberbauer, the man who arrested Anne Frank.
In 1980, he moved back to Israel to carry on his investigations, first working for the US Justice Department's Office of Special Investigations, tracking down war criminals who had fled to the States, before returning to the Simon Wiesenthal Center in 1986, becoming its chief Nazi hunter in Jerusalem.
Since then he has worked tirelessly to encourage governments and states to take action against those who managed to evade detection.
"People always ask me what's my job," he says. "So I say I'm one third detective, one third historian - to build the case against them - and one third political lobbyist. In many countries there is no political will to prosecute them and without that, there is no prosecution.
"Sometimes we work with local politicians and the media to try and raise the consciousness, to encourage governments that are reluctant, to do the right thing and chase these people."
But how did so many thousands manage to escape?
Following the end of the Second World War, Zuroff says, there were three groups of Nazis that managed to escape. The first were scientists and engineers with 'valuable skills, invited to the US for fear they might fall into Soviet hands'.
The second, much smaller, group were trained as spies by the West. The third - 'the largest by far' - were those from Eastern Europe who simply disappeared and enjoyed long lives.
"Simply by lying on their immigration forms they were able to fool the immigration authorities," he says. "Even though they had served with the Nazis and some of them played very important roles in the mass murder of Jews and other enemies of the Reich.
"They didn't even have to change anything on their forms; no one paid any attention. Very little was known about the role played by Eastern Europeans."
Tracking them down so many years after the war - with few brought to trial - can be physically and mentally tiring, says Zuroff.
But even when they are convicted, it can feel hollow.
"It's a very painful issue," says Zuroff. "I've been involved in more than 40 cases; some legal action was taken, either they were publicly exposed, indicted or made to stand trial. Very few though.
"There have been three successful prosecutions in Germany since the change in prosecution policy a decade ago - [John] Demjanjuk, Oskar Groening, and SS guard Reinhold Hanning - none of them have sat in jail after their conviction, so this is incredibly frustrating.
"All three of them died, Groening about three days before he was due to go to jail, while Hanning and Demjanjuk died during the appeal process.
"Obviously it's very painful when these people escape justice."
But one thing the father-of-four has learned over the years is that justice isn't always black and white.
He says: "Demjanjuk sat in prison in Israel for more than seven years, he sat in prison in the US, and in Germany, his case lasted more than 30 years and he basically had no life. He was punished, not perfectly, but he was punished.
"I went after Sándor Képíró - gendarmerie in Serbia in charge of thousands of people - and we basically ruined his life; we made it so that he was exposed.
"And sometimes the exposure and the revelations about what these people did are more painful to them than sitting in jail - in most cases their families don't have a clue what they did."
Inevitably some have slipped through the net. One of those was Aribert Heim. Known as Dr Death and the Butcher of Mauthausen, Heim was responsible for torturing and murdering countless prisoners during the war, injecting toxic substances directly into their hearts.
In 2008, Zuroff travelled to South America to look for him, pleading with the public, appearing on TV shows and offering a cash reward for information.
It eventually transpired that Heim had died in Egypt more than a decade earlier.
"Devastated is the word. Absolutely devastated," Zuroff shouts down the phone.
As the years trickle by, the fact is that age will inevitably get the better of justice, and those who brought so much pain to so many will die having not paid for their crimes.
As numbers dwindle, Zuroff's work becomes more focused on fighting 'Holocaust Distortion' and educating younger generations about the importance of remembering what happened all those years ago.
But despite a change of priorities, he says he owes it to the families of those who were butchered in death camps to find the people responsible - and says age should offer no protection.
"What keeps me going is the sense of obligation to the victims; to try and maximise justice," he says.
"I will give you one story that, for me, tells you everything. I helped bring one of the commanders of one of the worst concentration camps in Croatia to justice, a man named Dinko Šakić.
"We exposed him living in Argentina, had him extradited, and he was given the maximum sentence for murder - though not for genocide, that was a political decision.
"One of the most dramatic testimonies during the trial recalled a day some of the inmates violated disciplinary laws - so Šakić immediately ordered a roll call and had all inmates appear at the central square.
"He began walking up and down the rows and, at random, picking people out to be hanged in reprisal for the breach of discipline.
"One of the people he picked out was a doctor from Montenegro, Milo Bošković. When he was taken out of the line he said: 'I'm from Montenegro and my tradition doesn't allow me to be hanged.' Šakić took out his pistol and shot him in the head.
"At the trial, on the day of the verdict, a very tall, well-dressed man stops me and says: 'Listen, I have just one word to say to you; hvala', which means thank you.
"I had no idea who this guy was, so I asked someone. It was Milo Bošković's brother.
"Whoever believed in a million years that the day would come when Croatia would be independent and could bring murderers like Šakić to trial and convict him."
He concludes: "That story I take with me everywhere. That's my consolation."
Dr Zuroff's new book, 'Our People: Discovering Lithuania's Hidden Holocaust' is on sale now.
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