By Kathryn Westcott
If he is still alive, former SS medical officer Aribert Heim is 93 years old, but his age will not protect the alleged Nazi war criminal from justice, vows Nazi-hunter Efraim Zuroff.
Mr Zuroff is in South America this week for the launch of Operation Last Chance, a scheme to flush out the remaining Nazi criminals who took refuge there in the postwar years.
But the investigators are working against time: most of the alleged criminals are in their late 80s or 90s.
The operation, already under way in Europe, offers potential informants a reward of $10,000 for any information that results in the conviction of a Nazi criminal.
Heim is the number one target in South America and the total bounty for his capture is $448,000.
The Simon Wiesenthal Centre, which has decades of experience tracking down Nazis, says that if Heim were brought to justice, it would be the most significant World War II war crimes trial for three decades.
The Austrian-born physician is said to have murdered hundreds of inmates while serving as a doctor at Mauthausen concentration camp in Austria, where he earned the nickname "Dr Death".
Efraim Zuroff wants to pursue Heim to the grave
He is accused of killing Jews using exceptionally cruel methods. According to Holocaust survivors, he performed operations and amputations without anaesthetic to see how much pain his victims could endure.
Injecting victims straight into the heart with petrol, water or poison were said to have been his favoured method at Mauthausen. And when he was "bored", he apparently timed patients' deaths with a stopwatch.
After World War II, Heim practised medicine in the German town of Baden-Baden until 1962, when he was indicted as a war criminal and fled the country.
Over the years there have been alleged sightings of him in Egypt, Uruguay, Chile and more recently in Spain.
'Closer than ever'
Mr Zuroff says new information has led him to believe that he is still alive and living somewhere in South America.
Mauthausen concentration camp, where Heim was a medical officer
"It would be foolish to make any rash predictions, but I think we are closer than ever before [to catching him]," Mr Zuroff told the BBC News website from Argentina.
"Recent indications directed to me - I'd rather not say what they are - point to the direction [of Latin America]. We have been getting quite a bit of information, some of it has been worthless and some of it serious."
Other countries in the region that have signed up to Operation Last Chance are Chile, Brazil and Uruguay.
"We don't know how many Nazi war criminals are in those countries, but we think it's dozens, if not hundreds. We have expectations of catching all of them but if we only get Heim, it will be a success," says Mr Zuroff.
Heim's family say that he died in the 1990s. But in 2005, European investigators discovered a bank account in his name, which is reported to contain $1m. The fact that Heim's family had not claimed the money led them to believe that he was still alive.
Mr Zuroff is now in Chile, where he believes Heim's daughter lives. He is optimistic that the size of the bounty could prove crucial to his capture.
"In South America, this is a large sum of money. We have already seen that this approach can be successful. We saw it in the Schwammberger case," he says.
A reward proved crucial in the capture of Josef Schwammberger
The offer of a reward in the late 1980s led investigators to notorious camp commander Josef Schwammberger who was extradited from Argentina to end his days in a German prison.
"Maybe this could even convince Heim's daughter to finally reveal his whereabouts," says Mr Zuroff. "Surely she would prefer that he not be the subject of a headhunt by all sorts of people who want to get the money."
The man who took over from chief Nazi-hunter Simon Wiesenthal when he died in 2005, believes that recent political changes in South America have greatly improved the climate for unearthing former war criminals.
In Europe, however, he says he has encountered a frustrating lack of political will to bring alleged Nazi war criminals to justice.
The scheme, launched in 2002, is operating in Lithuania, Latvia, Estonia, Austria, Belarus, Germany, Hungary and Ukraine.
So far, it has yielded the names of 488 suspects from 20 countries, the Simon Wiesenthal Centre says, including Heim. Of that number 99 have been submitted to local prosecutors.
"This has led to three arrest warrants, two extradition requests and five solid cases, including that of Heim, that we hope will be brought to trial," says Mr Zuroff.
The strongest case in Europe is that of 94-year-old Milivoji Asner. According to the Centre, the notorious police chief of Pozega, Croatia, played a key role in the persecution and murder of hundreds of innocent civilians.
Following his exposure by Operation Last Chance in 2004, he escaped to Klagenfurt Austria, where he currently lives.
"The scheme has not been as successful as we hoped in the practical sense of achieving convictions," says Mr Zuroff.
But, he insists that the operation was having "enormous impact" on what he describes as a struggle for historical truth in post-communist Europe, where there is an issue of local complicity in the murder of Jews.
Heim's wanted poster shows him as he might look today. He may still have his distinctive duelling scar running down the right side of his face, from his mouth to the corner of his ear, or he may have taken the precaution of appearance-altering surgery.
If he is caught, some may argue that it is wrong to put an a frail old man on trial for alleged crimes committed more than half his lifetime ago.
To this, Mr Zuroff has only one thing to say: "The passage of time in no way diminishes the guilt of the perpetrators. Killers don't become righteous gentiles when they reach a certain age. And if we were to set a chronological limit on prosecutions, it would basically say you could get away with genocide.