A key component of the Endlösung was building several camps in which Jews could be murdered in large numbers using gas. As early as the summer of 1941, the head of the SS, Heinrich Himmler complained that the mass executions posed a heavy psychological burden for his men. In December 1939, experiments with gassing had been carried out in Germany on tens of thousands of handicapped people. In Poland, lorries were used for the first time to gas people in the autumn of 1941. During that time, the first experiments with hydrogen cyanide Zyklon-B were carried out in Auschwitz - which at the time was still only a concentration and labour camp. The Nazis decided to build extermination camps in Poland and a plan was drafted for deporting all the Jews from the occupied countries in Western and Southern Europe by train to 'the East'. In addition to Auschwitz-Birkenau, the largest and most significant German extermination camps in Poland were Sobibor, Treblinka, Majdanek, Belzec, and Chelmno. The remaining Jewish quarters, villages, and cities that had been spared so far, as well as the Jewish ghettos in Poland and other European countries were emptied of Jews who were deported to the extermination camps.