At Birkenau, Clinton stood at the site where hundreds of thousands of Jews from across Europe arrived in cattle cars and were "selected" for hard labor or immediate death in the gas chambers.

"This was cold, rational evil" that used "tools of technology and bureaucracy to exterminate millions of people in a deliberate effort at genocide and political conquest," Clinton said, visibly shaken.

"The lessons of this place should never be forgotten," she said.