As the Simon Wiesenthal Center prepares to publish its list of most-wanted suspected Nazis on Wednesday, the The Associated Press investigates the case of the concentration camp doctor who tops the list.

Karl Lotter, a prisoner who worked in the hospital at Mauthausen concentration camp, had no trouble remembering the first time he watched SS doctor Aribert Heim kill a man.

It was 1941, and an 18-year-old Jew had been sent to the clinic with a foot inflammation. Heim asked him about himself and why he was so fit. The young man said he had been a soccer player and swimmer.

Then, instead of treating the prisoner's foot, Heim anesthetized him, cut him open, castrated him, took apart one kidney and removed the second, Lotter said. The victim's head was removed and the flesh boiled off so that Heim could keep it on display.

"He needed the head because of its perfect teeth," Lotter, a non-Jewish political prisoner, recalled in testimony eight years later that was included in an Austrian warrant for Heim's arrest uncovered by The Associated Press. "Of all the camp doctors in Mauthausen, Dr. Heim was the most horrible."

But Heim managed to avoid prosecution, his American-held file in Germany mysteriously omitting his time at Mauthausen, and today he is the most-wanted suspected Nazi war criminal on a list of hundreds who the Simon Wiesenthal Center estimates are still free.

"Heim would be 93 today and we have good reason to believe he is still alive," said Efraim Zuroff, the Simon Wiesenthal Center's top Nazi hunter. He spoke in a telephone interview from Jerusalem before the center plans to release a most-wanted list Wednesday, and to open a media campaign in South America this summer highlighting the $485,000 reward for Heim's arrest posted by the center along with Germany and Austria.

According to an advance copy of the list obtained by the AP, the most wanted, after Heim, are: John Demjanjuk, fighting deportation from the U.S., which says he was a guard at several death and forced labor camps; Sandor Kepiro, a Hungarian accused of involvement in the wartime killings of than 1,000 civilians in Serbia; Milivoj Asner, a wartime Croatian police chief now living in Austria and suspected of an active role in deporting hundreds of Serbs, Jews, and Gypsies to their deaths; and Soeren Kam, a former member of the SS wanted by Denmark for the assassination of a journalist in 1943. His extradition from Germany was blocked in 2007 by a Bavarian court that found insufficient evidence for murder charges.

The hunt for Heim has taken investigators from the German state of Baden-Wuerttemberg all around the world. Besides his home country of Austria and neighboring Germany where he settled after the war, tips have come from Uruguay in 1998, Spain, Switzerland and Chile in 2005, and Brazil in 2006, said Heinz Heister, presiding judge of the Baden-Baden state court, where Heim was indicted in absentia on hundreds of counts of murder in 1979.

Thousands of German war criminals were prosecuted in West Germany after World War II. In the 1970s, Western democracies began a hunt in earnest for Eastern European collaborators who had fled claiming to be refugees from communism, and the end of the Cold War gave access to a trove of communist files in the 1990s.

"All of a sudden there was pressure on countries like Latvia and Estonia to put these people on trial," Zuroff said. "So two times in the past 30 years we've been given a tremendous infusion of new energy and new possibilities.

The Wiesenthal Center's previous annual survey counted 1,019 investigations under way worldwide. The number is lower this year and inexact because not all countries responded, but new investigations were up from 63 to 202, Zuroff said.

Still, a lack of political will in many countries, and what Zuroff called the misplaced-sympathy syndrome - reluctance to pursue aging suspects - has meant that few people have been brought to trial and convicted.

Lotter, the witness to Heim's atrocity, was in Mauthausen because he fought with the communists in the Spanish Civil War. His statement from the 1950 arrest warrant was viewed by the AP at the National Archives in College Park, Maryland.

Now that the necessary evidence is in place, numerous witness statements have been taken and Heim has been indicted, all that's left is to find him.

Born June 28, 1914 in Radkersburg, Austria, Heim joined the local Nazi party in 1935, three years before Austria was bloodlessly annexed by Germany.

He later joined the Waffen SS and was assigned to Mauthausen, a concentration camp near Linz, Austria, as a camp doctor in October and November 1941.

While there, witnesses told investigators, he worked closely with SS pharmacist Erich Wasicky on such gruesome experiments as injecting various solutions into Jewish prisoners' hearts to see which killed them the fastest.

But while Wasicky was brought to trial by an American Military Tribunal in 1946 and sentenced to death, along with other camp medical personnel and commanders, Heim, who was a POW in American custody, was not among them.

Heim's file in the Berlin Document Center, the then-U.S.-run depot for Nazi-era papers, was apparently altered to obliterate any mention of Mauthausen, according to his 1979 German indictment, obtained by the AP. Instead, for the period he was known to be at the concentration camp, he was listed as having a different SS assignment.

"This cannot be correct," the indictment says. "It is possible that through data manipulation the short assignment at the same time to the (concentration camp) was concealed."

There is no indication who might have been responsible.

Eli Rosenbaum, director of the Justice Department's Nazi-hunting Office of Special Investigations, was on the road and did not immediately respond to requests for comment.

The U.S. Army Intelligence file on Heim could shed light on his wartime and postwar activities, and is among hundreds of thousands transferred to the U.S. National Archives. But the Army's electronic format is such that staff have so far only been able to access about half of them, and these do not include the file requested by the AP.

Heim was relatively well-known, however, having been a national hockey player in Austria before the war, and there were plenty of witnesses from his time at Mauthausen.

Austrian authorities sent the 1950 arrest warrant to American authorities in Germany who initially agreed to turn him over, then told the Austrians, in a December 21, 1950 letter obtained by the AP, that they could not trace him.

What happened next is unclear, but in 1958 Heim apparently felt comfortable enough to buy a 42-unit apartment block in Berlin, listing it in his own name with a home address in Mannheim, according to purchase documents obtained by the AP. He then moved to the nearby resort town of Baden-Baden and opened a gynecological clinic - also under his own name, Heister said.

In 1961, German authorities were alerted and began an investigation, but when they finally went to arrest him in September 1962, they just missed him - he apparently had been tipped off.

Heim continued to live off the rents collected from the Berlin apartments until 1979 when the building was confiscated by German authorities.

Proof that he is alive may lie in the fact that no one has claimed his estate. Heim has two sons in Germany and a daughter who lived in Chile, but whose current whereabouts are unknown.

In Frankfurt, Heim's lawyer said he still officially represents the fugitive, but has not heard from him for 20 years and has no clue to his whereabouts.

Asked in a telephone interview if Heim was dead, Fritz Steinacker said only: I don't know.

Ruediger Heim, one of the sons, would not comment when telephoned at his Baden-Baden villa.

"All I can say is that it has been implied that I am in contact with my father, and that is absolutely false," he said. "The rest is speculation, and I can't enter into that."