Calling himself “a son of Germany,” Pope Benedict XVI prayed at the former Nazi death camp of Auschwitz on Sunday and asked why God was silent when 1.5 million victims, mostly Jews, died in this “valley of darkness.”

Ending a four-day pilgrimage to Poland, Benedict said humans could not fathom “this endless slaughter” but only seek reconciliation for those who suffered “in this place of horror.”

As on the rest of his trip, he walked in the footsteps of his Polish-born predecessor John Paul, who came to the camp in 1979 on his first visit to Poland as pope. John Paul died in April 2005 and is revered as a saint in his native country.