In 1942, he twice toured the ghetto disguised as a Jew and then sneaked into a camp near Belzec dressed as a Ukrainian guard.
Karski then undertook a journey to spread the word about the atrocities he discovered.
"I saw terrible things that will haunt me for the rest of my life," he said last week in a telephone interview from his home in Chevy Chase, Md.
Karski, now 82, will relay his story at the annual community observance of Yom HaShoah, or Holocaust Remembrance Day. The free event, sponsored by the S.F.-based Jewish Community Relations Council and the Holocaust Center of Northern California, will take place in San Francisco at 7:30 p.m. Monday, April 15, at Congregation Emanu-El, 2 Lake St.
After secretly touring the ghetto and the camp, Karski made his way to England and then to the United States. He met personally with scores of top officials including President Franklin D. Roosevelt.
He told them all of Adolf Hitler's plan to exterminate the Jews. He brought requests for the Allies to formally condemn the genocide and to save the Jews by issuing blank visas or passports.
He got the same reaction from everyone — disbelief or rejection. The Allied leaders told him that aiding the Jews was impossible.
"The decision was that no side issues should intervene with the basic war strategy," he said. "The Jewish tragedy was never…more than a side issue."
Even today, the re-sponse haunts Karski.
"I don't accept it. It's painful. But such is the world, such is humanity," he said in his still heavy Polish accent.
Karski has been honored by the Yad Vashem Holocaust memorial as a Righteous Gentile Among the Nations. He has been given honorary Israeli citizenship. He has received Poland's highest honor. And three books have been written about him.
Yet the retired Georgetown University government professor dismisses his long-ago mission to save the Jews as "nothing extraordinary."
"At that time, after what I saw, very simply I had to do it," he said.
As a result of his experiences during World War II, Karski has always supported Israel. For him, the existence of the Jewish state is the only guarantee against another Holocaust.
"At that time, during the war, the Jews were totally helpless. They had no country of their own, no government, no army…no voice. They had to rely on others, who were sympathetic or not sympathetic," he said.
In 1944, he published a book about the Nazis called "Story of a Secret State." But then Karski stopped talking about his experiences.
"I wanted to be normal," he said. "Do you know the overwhelming majority of the Jews who survived and who came to the United States also conducted themselves in such a way?…I wanted to forget what I saw."
He became an American citizen. He married a Jewish woman. He earned his Ph.D. and taught Eastern European and international affairs at Georgetown.
The silence lasted for three decades. Then in the late 1970s, filmmaker Claude Lanzmann, the creator of the epic documentary "Shoah," convinced Karski to tell his story.
Once he opened up, Karski realized he had important messages to spread about the Holocaust. Though gradually growing more infirm, he still speaks in public and gives interviews.
Today, he especially wants to set the record straight for young Jews.
"They are under the impression that everyone hated the Jews. It's not true," he asserted.
Karski readily acknowledges that the Jews were abandoned by governments, by religious institutions, by all existing social structures.
"But the Jews were not abandoned by all humanity," he said. "There were thousands upon thousands who tried to help the Jews."