Unlike at Auschwitz, where the cyanide-based Zyklon-B was used to exterminate trainloads of prisoners under the guise of "relocation", the camps at Treblinka, Bełżec, and Sobibór, built during Operation Reinhard (October 1941 – November 1943), used lethal exhaust fumes produced by large internal combustion engines. The three killing centres of "Einsatz Reinhard" were constructed predominantly for the extermination of Poland's Jews trapped in the Nazi ghettos. At first, the victim's bodies were buried with the use of crawler excavators, but they were later exhumed and incinerated in open-air pyres to hide the evidence of genocide in what became known as Sonderaktion 1005.