By Kathryn Westcott
Aribert Heim's crimes rank among the worst of the Holocaust, and after a hunt that has spanned almost half a century, Nazi-hunters believe they are closing in on him.
The search for the 94-year-old former SS medical officer has taken investigators from Germany all around the world. Besides his home country of Austria and Germany, where he lived after the war, tips have come from Uruguay, Spain, Switzerland, Brazil and Chile.
Nazi-hunters are now confident that Heim - who was known for his sadism as a doctor at the Nazi's Mauthausen concentration camp in Austria - is seeing out his twilight years near his daughter in southern Chile, or across the border in Argentine Patagonia, the region between the Andes and the south Atlantic.
Acting on information about Heim's whereabouts that he describes as "so significant that it has a high potential", Efraim Zuroff, the Simon Wiesenthal Centre's top Nazi-hunter, has travelled to the Chilean town of Puerto Montt, 657 miles (1,058 km) south of the capital Santiago, where he says Heim's 64-year-old daughter lives.
He refuses to divulge the nature of the recent information.
Heim's daughter has said that her father died in 1993 in Argentina, but a death certificate has never been produced. Neither she nor her two brothers, who live in Germany, have claimed the estimated $1.5m (900,000 euros; £750,000) that sits in a European bank account in Heim's name.
On his way to board the flight to southern Chile, Professor Zuroff - who has been hunting Nazi war criminals and their collaborators for nearly three decades - told the BBC News website that he believes he is closing in on Heim.
Efraim Zuroff says he has received significant information
He was in the country back in November, when the centre launched Operation Last Chance, its final attempt to find Nazi war criminals in South America.
A reward of 315,000 euros, or almost $500,000, is being offered jointly by the centre and the German and Austrian governments for information that will lead to Heim's arrest and prosecution by the German government.
Mr Zuroff is optimistic that the size of the bounty could prove crucial to his capture. He is now launching a newspaper campaign advertising the bounty in Chile and Argentina.
"By going closer to the area where we believe he may well be hiding, we are trying to increase the chances of finding someone with the critical information regarding his whereabouts," he said.
"In the past, money has exposed Nazi war criminals, such as Schwammberger in Argentina. This was a great success and we hope to add another one very soon."
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.
Mr Zuroff says he doesn't expect to find Heim immediately.
Mauthausen concentration camp, where Heim was a medical officer
"We're putting in place certain initiatives which could reveal his hiding place within a couple of weeks," he says.
"He would then be extradited to Germany to stand trial assuming that his health permits. Everyone is fairly confident that it would go smoothly and quickly.
"Germany has created a special task force to find Heim. He would be tried there and it would be the most important Nazi war crimes trial in the past 30 years."
The Austrian-born physician has been indicted by Germany on charges that he murdered hundreds of inmates while serving as a doctor at Mauthausen concentration camp in Austria, where he earned the nickname "Dr Death".
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 was 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.
'Passage of time'
One testimony from a camp survivor accuses him of cutting off the head of a murdered Jewish prisoner and boiling off the flesh to enable the skull to be used as an exhibit.
Stories like this abound. One claims that the doctor removed tattooed skin from one victim and turned it into a seat cover.
Born 28 June, 1914 in Radkersburg, Austria, Heim joined the local Nazi party in 1935, three years before Austria was annexed by Germany. He later joined the Waffen SS.
If Heim is still alive, he would have just turned 94. 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.
But Mr Zuroff is uncompromising: "The passage of time in no way diminishes the guilt of the perpetrators," he said in a previous interview with the BBC.
"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."
The centre believes there are dozens of Nazi war criminals and their collaborators in Latin America. The reward for information leading to their arrest has just been increased from $10,000 to $15,000.
But the investigators are working against time: most of the alleged criminals are in their late 80s or 90s.
They say that if they only get Heim, Operation Last Chance would have been a success.
In the next few days, Mr Zuroff will travel across the Andes to San Carlos de Bariloche in Argentina.
Nestled in Argentina's snow-capped Andes, overlooking a vast lake, this scenic ski town has long been a favourite resort for wealthy tourists. It was also known over the years as a haven for Nazis who fled Germany after World War II.
Former SS captain Erich Priebke, was discovered there in the early 1990s. He had been living there for almost half a century.
For the hunter, Mr Zuroff, this will be his last trip to South America on the trail of the war criminals. He acknowledges that the last push might fail completely. But, for the moment, he is optimistic that he is closer to his prey than ever.