President Barack Obama will honor a deceased former Polish underground officer Jan Karski, one of the first people to expose Nazi atrocities.

Karski, a Catholic, reported to the Polish government in exile and the Western Allies on the situation in German-occupied Poland in 1942 and 1943, especially the destruction of the Warsaw Ghetto and the existence of secretive Nazi extermination camps.

Speaking at the U.S. Holocaust Memorial Museum several days after Holocaust Remembrance Day – Yom HaShoah – the president said he will honor Karski posthumously with the highest American civilian honor, the Presidential Medal of Freedom. Karski died in 2000 in the United States, where he lived for decades.

"We must tell our children about how this evil was allowed to happen: because so many people succumbed to their darkest instincts, because so many others stood silent," President Obama said.

Karski was a clandestine officer of the Polish government-in-exile in London and infiltrated into the Warsaw Ghetto, where he saw starvation and death.

He also entered a Nazi transit camp near Warsaw by dressing as a Ukrainian guard. He was an eyewitness to Nazi atrocities, including beating and stabbing Jews and stuffing them into trains on their way to the gas chambers.

Karski warned U.S. President Franklin D. Roosevelt of the horrors of the Holocaust. Roosevelt has been widely criticized for having known of the Holocaust and not taking any action against it.