FRANKFURT For the few surviving inmates of Mauthausen concentration camp, one visitor in the autumn of 1941 left an indelible memory.
Tall and athletic, Aribert Heim was the camp doctor for only two months and the 27-year-old enjoyed his time in the Austrian town.
On one occasion, he picked out a prisoner passing his office. After checking his teeth, Heim persuaded him to take part in a medical experiment with the vague promise of release.
Heim killed the man with an injection of poison to his heart, later severing his head and using the skull as a paperweight.
Injections to the heart -- with petrol, water or poison -- were a favorite experiment of Heim's, who timed patients' deaths with a stopwatch.
Sometimes, out of boredom, he carried out operations without anesthetic, removing organs from conscious victims.
Heim was arrested after World War Two but he was later released and was soon practicing as a doctor again. He moved to Baden Baden, a small town in western Germany.
But survivors of Mauthausen did not forget the camp doctor who delighted in seeing the fear of death in his patients' eyes. Police were sent to re-arrest Heim. The night before they were due to call, he disappeared.
Now German prosecutors are on Heim's trail again. They believe he is still alive because his wife and children have yet to claim money he left in a Berlin bank account.
Their search is the last gasp of the post-war hunt for Nazi war criminals. Prosecutors in Germany and the Austrian government have contributed to a reward of 310,000 euros ($448,000) for information leading to Heim's capture.
"We will pursue Heim even if our search ends up at a gravestone," said one German police investigator, who asked not to be named.
The hunt has taken them from Spain to South America. On a visit to one town in South America, investigators ran a check to find local German men aged over 90. More than 300 names came up.
Efraim Zuroff, who helped restart the pursuit of Heim, is carrying on the work of one of the world's best-known Nazi hunters, Simon Wiesenthal.
Wiesenthal was instrumental in helping Israeli secret service agents bundle Holocaust planner Adolf Eichmann into a plane from Argentina, later to be hanged.
A Mauthausen inmate, Wiesenthal never forgot the strapping young doctor but he died before he could hunt Heim down.
"There is a serious effort being made to find Heim but that's certainly not the case as far as others are concerned," says Zuroff, speaking by phone from Israel.
"I think people are just tired. This is a subject which requires zeal. There are no political obstacles to prosecution in Germany but they do things in such a bureaucratic way."
Zuroff says many countries are opting for the "biological solution". "In a few years these people will die and with their deaths, the problem has been solved. If these countries just wait it out, then they will spare themselves enormous expense and unwanted attention and be done with the problem."
Later this year, Zuroff will make a final tour of Nazi hideouts, where hundreds of war criminals are believed to be living out their twilight years, before he retires.
Serge and Beate Klarsfeld, who spent decades chasing those in the French establishment who organized purges for their Nazi masters, have also retired.
Serge Klarsfeld still vividly recalls a crucial discovery in the search for evidence against Klaus Barbie, the so-called butcher of Lyon, whom they tracked to Bolivia over 30 years ago.
"In among many boxes full of uninteresting documents, I discovered the original telefax he had sent from Lyon to Paris to inform the Gestapo he had arrested 44 children on April 6, 1944," Klarsfeld said by phone from Paris. "They were all killed.
"It is not like in the novels where the criminals like women and alcohol. They had good family lives. They could easily switch from one side of their lives to another, like Dr Jekyll and Mr Hyde."
But for the 72-year-old Klarsfeld, the hunt is now over. "The people who were around during the second world war are vanishing. And their children are also vanishing.
"I always thought that if all of those criminals could die on the same day, it would be wonderful. When I was trying to trace someone I was always hoping that they would not be alive."
(Reporting by John O'Donnell; editing by Robert Woodward)