I recently learned something interesting - the gas van was apparently invented in the Soviet Union in the late 30s, shortly before the Nazis started using similar execution methods.

Google Translate says "...he was following the instructions of the NKVD Ministry of Defense and that without them it would be impossible to perform so much the number of executions to which the arrested were sentenced by three threes at a time"

See: https://www.kommersant.ru/doc/1265324