The first death camps were Treblinka, Bełżec, and Sobibór. They were built during Operation Reinhard (October 1941 – November 1943), whose goal was to kill all of Poland's Jews. Trains full of people were sent to these camps, and were murdered as soon as they got there. At first, the camp's guards used carbon monoxide to kill the prisoners. They then buried their bodies in mass graves. Later, when there were too many bodies to bury, they started burning the bodies. So they could kill people and get rid of their bodies more quickly, the Nazis built gas chambers and crematoria in Treblinka and Bełżec. At Bełżec, they also started using Zyklon-B poison gas to kill people.