By order of Heinrich Himmler, the crematoria and gas chambers at Birkenau were destroyed by the SS in order to hide evidence of genocide. Nothing more than the bases of Crematoria IV and V can be seen: the floor plans of both facilities are indicated by bricks laid out across the concrete foundations, and Crematoria II and III are in ruins. Professor Robert Jan van Pelt labels Leuchter's comment that the facilities have not changed at all since 1942 or 1941 as "nonsense".