Simon Wiesenthal knew this about the demons of the past: They often hide in plain sight. Once, while in line at a Washington, D.C., airport, Wiesenthal began a noisy, nose-to-nose confrontation with another traveler, who was eventually barred from the plane. "I said, 'Simon, what's the problem?'" recalls Rabbi Marvin Hier, who runs the L.A. center that bears Wiesenthal's name. "He said, 'This man is on my list as a suspected war criminal!'"
From 1945, when he was liberated from the Mauthausen concentration camp at 36, until his death Sept. 20 in Vienna, Wiesenthal never rested in his pursuit of the murderers of 6 million Jews and millions of others during World War II—among them his mother and 88 relatives. He was credited with helping track down 1,100 war criminals, including some of the most infamous (see box). Recalls Hier: "He said, 'If we don't do anything about evil, that will encourage future perpetrators.'"
Born in what is now Ukraine, Wiesenthal trained as an architect. But after the war he pledged himself to tracking Nazis, with the support of his wife, Cyla, a fellow survivor, who died in '03. (They had one daughter.) A small Vienna office grew into a worldwide network dedicated to the Holocaust. In his later years, Wiesenthal also worked to raise awareness of other war crimes, such as the Rwandan genocide. That, he believed, was a survivor's burden. "A last witness, before he leaves this world, has an obligation to speak out," he told PEOPLE in 1988. "My work is a warning for the murderers of tomorrow."