[3]: 253 Following the invasion of Poland he was sent to Litzmannstadt as part of this group in order to perform measurements on ethnic Germans and determine whether they were suitable for resettlement programmes in the east or simply for forced labour.
[4] In 1942 Fleischhacker was, along with Heinrich Rübel, chosen by Bruno Beger to work with him in an SS project to determine the racial type applicable to the Mountain Jews of the Caucasus, a group that did not readily fit the Nazi's existing ethnic paradigms.
[3]: 253 The following year he worked again with Beger, this time at Auschwitz concentration camp where again his duty was to measure physical features of the inmates in order to determine to which race they belonged.
After the Second World War Fleischhacker was held in various internment camps until appearing before the board of arbitration in 1948, where he was designated Mitläufer - not a fully committed Nazi but one who nonetheless went along with Nazism.
Ultimately the case against him was dismissed the following year as there was insufficient evidence to prove that he knew he was involved in extermination and only Beger was convicted.