sentences:  In December 1939 and January 1940, gas vans equipped with gas cylinders and a sealed compartment had been used to kill the handicapped in occupied Poland.As the mass shootings continued in Russia, Himmler and his subordinates in the field feared that the murders were causing psychological problems for the SS, and began searching for more efficient methods.In December 1941, similar vans, using exhaust fumes rather than bottled gas, were introduced into the camp at Chełmno, Victims were asphyxiated while being driven to prepared burial pits in the nearby forests.