When the Germans launched their attack on the Soviet Union they began a new chapter in the holocaust, the introduction of the Einsatzgruppen, special units whose job it was to follow the regular army, round up the Jews and kill them. The units were made up of regular Germans often police officers. Once an area was secure, they would round up the Jews in a town and take them to a nearby forest or ravine, and then kill them. The units were under the command of the SS but received logistic support from the army. While the exact number of Jews killed by the Einsatzgruppen is not known it is estimated that the number was in the hundreds of thousands. After a number of months of killing Jews by shooting them it was decided that the psychological toll on some of the men might be too high and mobile gas vans were introduced to kill the Jews. It should be noted that no men were required to be part of the Einsatzgruppen, and as far as has been determined no sanctions were ever levied against a member of the unit who ask not to take part in the killing or join another unit.