Used canisters that contained Zyklon-B, the deadly poison used in the Auschwitz gas chambers.
The words of Pope Benedict at Auschwitz:
"The rulers of the Third Reich," he said in a paragraph that deserves to be read at least a thousand times, "wanted to crush the entire Jewish people, to cancel it from the register of the peoples of the earth. … Deep down, those vicious criminals, by wiping out this people, wanted to kill the God who called Abraham, who spoke on Sinai and laid down principles to serve as a guide for mankind, principles that are eternally valid. If this people, by its very existence, was a witness to the God who spoke to humanity and took us to himself, then that God finally had to die and power had to belong to man alone — to those men, who thought that by force they had made themselves masters of the world. By destroying Israel, by the Shoah, they ultimately wanted to tear up the taproot of the Christian faith and to replace it with a faith of their own invention: faith in the rule of man, the rule of the powerful."
And Kaddesh for those who died: