During the Great Purge in the Soviet Union, an NKVD officer Isaj D. Berg used a specially adapted airtight van for gassing prisoners to death on an experimental basis.[17] The prisoner were gassed on the way to Butovo, a phony firing range, where the NKVD executed its prisoners and buried them.[18] According to testimony given by NKVD officer Nikolai Kharitonov in 1956, Isai Berg had been instrumental in the production of gas vans.[19] Berg had become chief of the administrative economic department in Moscow’s NKVD in the summer of 1937.[20] In October 1937 he was charged with the supervision of the Butovo firing range.[19] Berg had to prepare Butovo for the mass execution of people from greater Moscow and to ensure that these executions would take place smoothly.[21] According to testimony given by Fjodor Tschesnokov, a member of Berg’s execution team, in 1956, trucks were used, which were equipped with valves through which the gas could be directed inside the vehicles.