Every day, every week, Dr Efraim Zuroff receives another call, another tip-off regarding the identity, whereabouts and health status of former Nazi war criminals. To his dismay, the information is not always sound. But he checks it all anyway, with well-rehearsed diligence, in the hope that, some day, the men and women responsible for the Holocaust will face justice. His biggest fear? "That old age will catch up with them before me, " he says. Zuroff is a Nazi -hunter. Although he doesn't particularly care for the term, preferring "truth warrior", neither does he worry about the label. "It's what I do. I want to get the bastards. I want to bring them to justice."

I visit Zuroff in his cramped office in Jerusalem on a bright afternoon in early Spring. The walls are whitewashed. Pictures and photographs hang on one side; books on the Holocaust and Jewish history sit on the other. His desk is a labyrinth of papers, files and documents. A plain-speaking New Yorker, Zuroff is the Jerusalem director of the Simon Wiesenthal Centre (SWC). He took over from Wiesenthal himself, who died last year, aged 96, in Vienna . A survivor of five Nazi camps and seven other prisons, Wiesenthal spent the rest of his life - more than 60 years - pursuing Nazi war criminals and bringing them to justice. Zuroff, who works virtually alone, has taken on Wiesenthal's mission - and the onerous task of trying to track down and bring to trial as many surviving Nazi criminals before they die too. "Listen, it's not a personal war between me and the suspects, " he says. "I try to not
let this overshadow and overtake my life. But I'm very serious about this."

Zuroff has a crop of grey hair and long features, with wrinkles cascading from the corners of his eyes. His voice is forceful and direct, and virtually every sentence he speaks begins "listen", followed by a prolonged explanation featuring smiles and rolling eyes like he's scored some irrefutable point. In the age of the soundbite he is that most endangered of species: someone who speaks his mind. There is no disguising the taut, crackling energy that spills out of him even when he's sitting down. He is a tall but trim basketball-loving academic; a 57-year-old whose interest in Nazihunting began in 1978 following a chance meeting with Wiesenthal at the screening of a film about war criminals.

After completing his doctoral thesis on the Holocaust for the Hebrew University of Jerusalem, he emigrated to Israel permanently in 1980, beginning work as an investigator of Nazi war criminals for the US Justice Department's Office of Special Investigations. Since 1989 he has worked within the framework of the Simon Wiesenthal Centre, tracking dozens of criminals around the world. With the kind of schedule that would make a cabinet secretary blanch, each year he issues a report to various countries on how they deal with the search for Nazi criminals living in their jurisdiction.

While the death of Wiesenthal, whose work is believed to have brought some 1,100 Nazi war criminals to justice, appeared to herald the end of the search, Zuroff insists he still has a crucial role to play. And he argues that the passage of time in no way diminishes the guilt of the perpetrator. "The fact that someone reaches a certain chronological age is no reason to give him a pass, " he says. "The age is arbitrary. Seventy-five, 80, 85: what, they can get away with genocide? These are the last people on earth who deserve any sympathy. They don't deserve the right to enjoy their grandchildren, prune their roses and walk their dog. They deserve to be in the slammer, and the sooner the better."

In 2002 the SWC set up Operation Last Chance in the Baltic states and eastern Europe, in the hope that ageing war criminals would be caught with the help of dollars10,000 rewards for tips leading to prosecution and conviction. The money was put up by a Miami-based Jewish charitable foundation. So far the effort has generated information about local collaborators in Croatia , Romania , Lithuania , Latvia , Estonia , Poland and Austria . Referred cases are judged on whether the information regarding the crime is reliable, whether the suspect is alive and healthy, and whether he or she has been prosecuted in the past.

"The reward generates interest in the media, " says Zuroff, shrugging half-hopefully, half-pessimistically. "It's simply a gimmick, pure and simple, to get public attention. We get information all the time, but very little of that information is serious. I need serious evidence regarding what they did during the war. But what I have to fight for, and [what] is not assured, is first of all the accuracy of the historical record. We have countries in eastern Europe who would like nothing better than to say, 'Nasty Germans and Austrians came to our country and killed our Jews and that's the Holocaust.' That's total bullshit. This is a very important part of what I'm doing. This is the package. The package is much wider."

When I ask Rena Quint what she remembers most about the Bergen-Belsen concentration camp, the answer takes me by surprise. "The smell of the soup, " she says. "Old potato peels and old vegetables. It was the most delicious thing because it was the only thing we had. But people died eating this. Sometimes I pass a restaurant and there is that old smell of dishwater and slops. That's the smell. It's garbage. But that's what we ate." She pauses. The particularity of memory she attributes to simply trying to take something positive from the abject horror around her. "You don't forget smells and you don't forget feelings and sensations. The pictures of the dead bodies: well, you think, 'How could anyone do that?'" Quint looks away very briefly, her voice suddenly more fragile than a spider's web. It's the first time during our conversation that this eloquent voice has wavered.

Quint is an immaculate, precise woman who has just returned from her morning swim. She is 70. On a table in her handsome apartment in Jerusalem are documents detailing her life: dates, birthplaces and photographs. Factual records to prove she existed. Most of the details are simply reminders, of who she was and who she is. She was born in a Polish town called Piotrkow - which, in 1939, became the site of the first Jewish ghetto established in Poland by the occupying German forces. She fingers the papers in front of her and begins telling me random, transfixing facts about her life. She lived with her mother, father and two brothers. Her name then was Fredja Lichenstein. Her nationality was Polish. Her mother's maiden name was Messer. She spent one year in a ghetto with her father pretending to be a boy. "I have all this now to prove who I am." She looks at the papers on the table. The papers
say she was in Bergen-Belsen at a date not indicated. But she was liberated in April 1945. Then she was in a transit camp and sent to Sweden , aged about nine. From Sweden she went to America . "No other information could be obtained in respect of my parents, " she says. "Also none could be found about my brothers, Josef and David."

In 1989 she went back to Poland and found where she once lived. She found out she was born in December of 1935. Until a few years ago she believed it was 1936. "I found out my father was brought to Buchenwald [a concentration camp] on January 1, 1945. My mother was taken to Treblinka."

When Piotrkow became a ghetto, the Lichensteins didn't have to move out of their house, but Jewish people in surrounding neighbourhoods were moved in. People began dying because they were starving. Only those who were working as slave-labourers for the Germans, like Quint's father, survived. "I just remember two happy things about my childhood, " she says. "One that I had two brothers and they used to pull me on a sled. And on Saturdays my mother would make a big pot with potatoes and meat and take it to the bakers. The next day the food came back all baked."

When the people in the ghetto weren't dying fast enough, German soldiers and Ukranian helpers took them away. Some were taken to a forest and shot. Others were put on cattle cars, rounded up and taken to Treblinka, where they were later gassed. Their bodies were burned on open fires. Quint was in a cattle car with her mother and brothers. Her father had been kept to work in a factory. "There was a man on one side of the cattle-truck door who worked with my father, " she recalls. "He shouted, 'Run.'" That was the last time she saw her mother and brothers.

The man took her back to her father, who dressed her as a boy. Instead of Fredja she became Froim, and began working alongside the men. Her diet consisted of flour and sawdust baked into bread. One day there was an order that the factories were going to be closed and they were again taken to the cattle cars. There was an order for men and women to be separated. The men went to Buchenwald and the women, eventually, to Bergen-Belsen . "My father knew that in the camps, if I was undressed, they would know I was a girl. He met a schoolteacher and asked if she would take care of me, and she became my new mother. That was the last time I saw my father."

Zuroff was born in the US in 1948 to a Jewish-American family. Although he was not directly affected by the Holocaust, his grandfather was the oldest of six boys born in Lithuania . As a young man, his grandfather went to America , while one of his great-uncles went to Scotland . He still has family in Glasgow . Two went to South Africa and one to Palestine . The sixth was killed in the Holocaust. "I was named after him. He was killed in Lithuania . Ironically a great deal of the work I've done has been on Lithuania , including trying to catch Anton Gecas." Zuroff leans back in his chair and sighs. "That case was one of the most important I've dealt with."

Gecas was accused of killing more than 30,000 mostly Jewish civilians during the Second World War. He died in September 2001 in an Edinburgh hospital, aged 85, after suffering a series of strokes. News of his death was met with anger and frustration by those who had been pressing the Scottish Executive to speed up the extradition process after Lithuania formally requested it in January 2001. In February 2001 a warrant was issued for Gecas's arrest by a court in Lithuania after prosecutors in Vilnius received information from the US Justice Department enabling them to proceed with a case against him. A further arrest warrant, this time in the UK , was issued in July 2001 at the request of the Lithuanian government - but was never issued because doctors refused to allow police officers to do so.

Zuroff, who first uncovered Gecas in the mid-eighties, is still angry that he never got to see him jailed. "Personally, it was the most frustrating case imaginable. Listen, the fact he was buried under a false name gave me a lot of satisfaction. When I think of all the people he killed who are buried in mass graves and whose families had no graves to mourn at, then I'm happy. But it's still a major frustration."

Perhaps Zuroff's most successful investigation so far was the one that culminated in the conviction of Dinko Sakic, the last known living commander of a Nazi death camp. In wartime Croatia , Sakic was believed to have been personally responsible for the slaughter of 2,000 people. "It was the proudest moment of my life, " Zuroff says, smiling. "Listen, this was a man who confided to journalists that he wasn't allowed to finish his job in the camps. He deserves all he's got."

Zuroff is now building a dossier on a suspected former Nazi war criminal living in Scotland . He is reticent about revealing too much of the case until he has more evidence. "We received information on a suspect who allegedly served in the Hungarian gendarmerie during World War II, " he explains. "The gendarmerie were the ones who rounded up the Hungarian Jews to put them into ghettos and then deported them to the Auschwitz death camp, where they were murdered. His name has suddenly surfaced in an important investigation linked to some very serious crimes, but this still has to be fully clarified before we make any accusations."

Between 30 and 50 soldiers who served in the Nazis ' notorious Galizien division are living in Scotland , according to Dr Stephen Ankier, a leading Holocaust researcher in the UK . Last year, newly released government documents revealed that 1,500 members of the unit - responsible for horrific war crimes in Poland and Ukraine - were brought to Scotland in 1947 and imprisoned in various camps. Ankier, whose Jewish parents fled from Poland to Britain in 1936, has previously urged the Metropolitan police to launch an investigation to identify any war criminals among the former SS men and bring them to justice. Last week he passed a dossier, including information about an ex-SS captain living in England , to the police. "The dossier has the names of lots of Ukranians, which may or may not correspond with the names of Galizien names already in the hands of the police, " he says. "I'm hoping they will
look at it and investigate it. But I'm extremely frustrated that it has taken so long to make any progress whatsoever." The UK War Crimes Unit was disbanded in 1999 but there is believed to be a dedicated team of around eight police officers still looking into war criminals in the UK .

At the top of Zuroff's wanted list, however, is 92-year-old Milivoj Asner. According to Zuroff, Asner is the former Nazi criminal he is most confident about catching. "It's an uphill battle but very important. Asner is the one we are closest to." On the subject of Asner's age, he retorts: "When people ask me why I'm after these old men I tell them that if I were chasing the person who murdered their grandfather the question wouldn't be asked. Each one of the murderers I chase murdered somebody's grandfather." According to Zuroff there are 1,252 ongoing SWC investigations in 16 different countries.

Perhaps the youngest Nazi -hunter in the world is Alen Budaj. And without his obduracy, Milivoj Asner might still be living the last few years of his life in obscurity. According to the 27year-old Budaj, who lives in Croatia , Asner served as police chief of the city of Pozega during 1941 and 1942. He is alleged to have stood by as the Ustashe, a powerful fascist movement in Croatia , looted and burned the Pozega synagogue, then established a detention camp where more than 300 detainees were killed. Asner allegedly oversaw the deportation of hundreds of Jews, Serbs and gypsies to the concentration camps. Most of the males went to the notorious Ustashe camp at Jasenovac, not far from Pozega.

Budaj, who has papers ordering the deportation of Serbs and Jews from Pozega allegedly signed by Asner, began researching his family's Jewish roots in 1997. Vladimir Bunjevcevic, one survivor from his home town, told him that Asner, who was believed to have died in a prison camp at the end of the war, was in fact living in Croatia , not far from Pozega. He was said to be in Daruvar, where he was born in 1913.

"I started to investigate and found out that he was actively leading his ultra-nationalistic political party, the IHSS [the Authentic Croatian Peasant Party], " says Budaj. He discovered that Asner had never been brought to trial, despite being pronounced as a war criminal by the Yugoslavian commission for the investigation of war crimes in 1946. Subsequently, Budaj discovered an anti-Jewish directive allegedly written by Asner in the Croatian national archives, along with other incriminating information, and passed it on to Operation Last Chance.

According to Budaj, in May 1945 Asner had escaped to Austria . He changed his name, obtained citizenship and started a new life in the southern city of Klagenfurt . He remained there, living prosperously, for the following 50 years, before returning to Daruvar in the 1990s. "In May 2004, " says Budaj, "I sent the completed file to the Simon Wiesenthal Centre in Jerusalem as a fully documented case." On June 30, 2004, Efraim Zuroff reported Asner to the Croatian attorney general. Despite the guarantees of the Croatian president, Stjepan Mesic, Asner escaped on the same day to Austria . Austria has not prosecuted a Nazi war criminal since the 1970s.

Asner was indicted in 2005 in Croatia for crimes committed during the Second World War, but has not yet been brought to trial. He is currently residing in Klagenfurt - and, since he is an Austrian citizen, the nation's government refuses to extradite him. " Austria has already said that 'some of the war crimes' of Milivoj Asner could be outside the limitation period for instituting legal proceedings, " says Budaj.

Despite Budaj's claims that he and others involved in the Asner case have received death threats, his work will continue. "I will do anything to bring Asner to justice, " he says. "It is not enough to know only the names of all victims; it is equally important to know the names of all perpetrators. I am obliged to do that for the victims, as it is stated on the monument in the Polish death camp Chelmno. 'We are smoke and dust and ash now, but you, dear friend, are obliged to find our murderers.' I cannot let Milivoj Asner die peacefully. I want a just punishment for him."

Milivoj Asner's wife picks up the telephone at their house in Klagenfurt . I explain the nature of my call. "He will speak with you tomorrow, " she says. Her voice is tired but authoritative. The old man picks up the telephone anyway, explaining that we must speak in either German, Croatian or Italian. "Tomorrow, " his wife says again, then hangs up. The following day I call, a German translator beside me. She explains to Asner what I am looking for. Immediately he tells her he is not interested. We call back several times but he simply puts down the phone.

Operation Last Chance. Zuroff admits that the name itself signals the future of Nazihunting. "There are only a few years left to get them, " he says with a pragmatic shrug. He knows time is running out. The quest of a lifetime is not yet over for Dr Efraim Zuroff, even though there are those who believe it should be. "I don't have to carry a burden of the [Holocaust] memory, " he says, "but what I did do, and what I do, is raise an awareness of the role played by collaborators. Simon Wiesenthal focused mainly on the Germans and the Austrians, the main initiators. I focused for many years on the helpers. But it's the last chance. That's why it's called that. It's the last chance." Asner will be 93 in April. Time is the enemy. And the clock is ticking regardless.

Rena Quint's life, meanwhile, changed for ever in Poland . She never saw her father again. "I believe he was killed in Buchenwald . I don't even remember what my own mother looked like, or my brothers. Five women who one by one became my 'mother' in the camp were also killed. Every day in Bergen-Belsen we would pull out the bodies of those who had died. Over fences, in pits. I accepted it as natural. When the British came in they just used bulldozers to push the bodies into pits." In April and May of 1945, British newsreels showed thousands of bodies being bulldozed into mass graves at Bergen-Belsen , and the world was suddenly introduced to the horrors of the Third Reich. Most of those inside were walking skeletons. Those who could not walk lay dying in their huts. Around 14,000 people died there after its liberation.

After a makeshift hospital, a displacement camp in Sweden and a journey to the US , Quint was finally adopted by a Jewish family from Brooklyn . They gave her clothes and food and she became their daughter, taking on a new name. "We never talked about the Holocaust. I had started a new life." So many years have passed since she lost her family but, surprisingly, she has few nightmares. "I guess when I was first growing up, " she concedes, "but the human tendency is to forget things. I think that hatred hurts everybody and war hurts everybody and until we learn to live with each other the world will be the way it is right now. But we should really try to make a different world than we have."

Quint believes in the work of the Nazihunters. She believes no-one should forget the Holocaust and the things that happened during it. "It's important to tell the story, " she says. "It's a good thing to keep after these people. It's very important for the world to continually know that it happened. Some of them were doctors, fathers and wives, dancers - normal people, but they left that part of their life. And they were able to kill. I don't know that I'll ever understand it. You also can't live with it all the time. But you can't forget." She looks away. "Years ago, but I don't do it any more, I used to look at the faces of men and say, 'Could he be my father, could he be my brother? After a while you know it's not."

A single tear wells in her eye. She gets up and shows me some photographs. The smiling faces of a whole new generation of Jews. "Hitler tried to wipe out all the children and here I am with 21 grandchildren and they will continue." There is no prosthetic for an amputated spirit, but new memories will always replace old.